Fabrication and machining of ceramic composites

Materials and Manufacturing Processes

ISSN: 1042-6914 (Print) 1532-2475 (Online) Journal homepage: http://www.tandfonline.com/loi/lmmp20

Fabrication and machining of ceramic composites — A review on current scenario Mandeep Singh Rayat, Simranpreet Singh Gill, Rupinder Singh & Lochan Sharma To cite this article: Mandeep Singh Rayat, Simranpreet Singh Gill, Rupinder Singh & Lochan Sharma (2017): Fabrication and machining of ceramic composites — A review on current scenario, Materials and Manufacturing Processes, DOI: 10.1080/10426914.2017.1279301 To link to this article: http://dx.doi.org/10.1080/10426914.2017.1279301

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Date: 16 February 2017, At: 20:23

MATERIALS AND MANUFACTURING PROCESSES http://dx.doi.org/10.1080/10426914.2017.1279301

Fabrication and machining of ceramic composites — A review on current scenario Mandeep Singh Rayata, Simranpreet Singh Gillb, Rupinder Singhc, and Lochan Sharmad a

Research Scholar, Faculty of Mechanical Engineering, IKG Punjab Technical University, Kapurthala, India; bDepartment of Mechanical Engineering, Beant College of Engineering and Technology, Gurdaspur, India; cDepartment of Production Engineering, Guru Nanak Dev Engineering College, Ludhiana, India; dIndian Institute of Technology, Jodhpur, India ABSTRACT

ARTICLE HISTORY

Ceramic matrix composites (CMCs) are the best-suited material for various engineering application due to their superior properties. The different processing methods involved in the fabrication and machining of these CMCs are a center for attraction to researchers and industrial society. This review article primarily focuses on the development of different processing methods and machining methods for ceramic matrix composites since the last few years. Out of these fabrication methods, powder metallurgy emerged as a most promising and cost-effective technique. In addition, electric discharge machining (EDM) has proved to be time saving, cost effective, and capable of machining complex shapes in composites. At the end, challenges in the processing and machining of ceramic matrix composites have been identified from the literature, and further benefits of microwave sintering and electric discharge machining of materials have been addressed in the paper.

Received 19 August 2016 Accepted 18 December 2016

Introduction Ceramics as a first man made material [1] have high hardness and strength, chemical inertness, low thermal conductivity, and good oxidation and corrosion-resistance properties [2]. However, for monolithic ceramics, it is not possible to mutate their various properties as per the requirement of different engineering applications due to their inherent brittle nature, poor electrical conductivity, and dubious mechanical properties. Thus, the main aim for making ceramic matrix composites (CMCs) is to overcome those scarcities that can be a dilemma for their use in various engineering application. Composites are generally made up of two or more materials in which one is known as matrix and other as filler material or reinforced material [3]. CMCs refer to the materials having good resistant properties to thermal shock, wear-resistant properties, high temperature creep behavior, and good fracture toughness. The use of filler material or reinforced material is to counteract the crack propagation growth due to redistribution of stresses in the adjacent areas to crack tip [4]. CMCs are the most favorable material used for different engineering application in hard and rough environment and are mostly used in conditions involving high-temperature, thermal shocks. These are generally used in automotive gas turbines, aerospace application, cutting tool inserts, carbon brakes, heat exchangers tubes, and many more [5,6]. The general reinforcements used for composites are titanium carbide (TiC), titanium nitride (TiN), boron nitride (BN), carbon nanofibers (CNF), carbon nanotubes (CNTs), graphene, carbon powder, etc. While fabricating CMCs, the most common issues encountered by different researchers were the degradation of

KEYWORDS

CVD; drilling; electrodischarge; grinding; manufacturing; microwave; powder; ultrasonic

reinforcement at elevated temperature in case of hot pressing and spark plasma sintering, presence of thermal stresses, poor dispersion of CNT, and difficult-to-make accurate and complex shapes [7–11]. So, need for further processing is required whether by means of conventional or nonconventional machining methods. Grinding and drilling are the most commonly used conventional methods for machining ceramics and their composite. Machining of ceramics and their composite by conventional methods is quite difficult and time consuming [12–15]. So to develop a cost-effective machining method for CMCs, industry switched to modern machining methods that include ultrasonic machining (USM), abrasive jet machining (AJM), abrasive water jet machining (AWJM), laser machining, electric discharged machining (EDM), etc. Material removal phenomenon in modern machining methods incorporates mechanical abrasion, chemical dissolution, melting or evaporation, and electrochemical dissolution [16–18]. In this paper, an attempt has been made to review the literature that deals with the various processing methods and machining methods available for ceramic and its composites.

Materials and Methods Ceramic matrix composites can be fabricated by various methods depending on the requirement and application of materials as reported by various researchers [19–110]. Figure 1 depicts the most commonly used processing techniques for ceramic matrix composites. The adoption of a specific processing route for the fabrication of CMCs depends on many factors that include required size and geometry of composite, operating

CONTACT Simranpreet Singh Gill [email protected] Department of Mechanical Engineering Beant College of Engineering and Technology, Gurdaspur-143521, India. Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/lmmp. © 2017 Taylor & Francis

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Figure 1. Processes used for fabrication of CMCs.

temperature, type of reinforcement, application of CMCs, and many more. In solid phase processes, powder metallurgy and slip casting are the well-defined methods. Sol-gel, Polymer infiltration pyrolysis processing methods come under liquid phase processes. Reaction bonding, chemical vapor deposition, and chemical vapor infiltration are included in gas phase processes [20,21]. Gas phase reaction methods include a chemical reaction between the reinforcement and the matrix phase under controlled conditions, and these methods generally include reaction bonding, chemical vapor deposition, and chemical vapor infiltration process. Different researchers reported the benefits of using these techniques such as lower processing time, temperature as well as the capabilities of producing near net shape, high densification rate, less energy required for nucleation and growth, and better resistance properties [19–25]. Li et al. [26] confers that the increase in temperature during sintering from 1450°C to 1700°C decreases the bulk density and compressive strength of material during reaction bonding. Thuault et al. [27] used single mode microwave cavity to obtain pore-free and homogeneous composite. Zhu et al. [28] fabricated a porous SiC ceramics by preceramic polymer reaction bonding and concluded that as the weight percentage of preceramic polymer polycarbosilane (PCS) increases, the porosity of material decreases and the fracture toughness increases as shown in Figs. 2 and 3. A novel reaction bonding technique known as meltinfiltration has been adopted by Thostenson et al. [29] for

Figure 2. Graph between weight percentage of PCS and porosity [28].

Figure 3. Graph between weight percentage of PCS and fracture toughness [28].

SiC/CNT composite. By using this technique, proper distribution of reinforcement was ensured. (Fig. 4), which leads to better electrical conductivity properties. Open porosity of mullite-bonded porous silicon carbide ceramic increases with the increase in graphite content but decreases with sintering temperature and forming pressure. Figure 5 shows the formation of open pores due to gaseous oxidation products [30–32]. Due to increase in sintering temperature mullitization increased, which leads to the improvement in flexure strength as shown in Fig. 6 [30]. In 1998, Samanta et al. [33] used a novel technique known as Intermediate Gel Formation to counter the oxidation phenomenon of SiC during reaction bonding. Li et al. [34] fabricated ceramic composite with an alloy FeMo at different temperature and found that the alloy has significant effect on the physical properties of ceramic composite. A similar study has been done by Zhu et al. [35] by the addition Si powder and LiO2 in reaction bonded silicon nitride. The results reveal that the increase in the purity of Si powder and presence of LiO2 decreased the thermal conductivity regardless of Al impurity. Value for high fracture toughness was about 11 MPa m1/2 for purest fine Si powder but four point bending strength was higher in case of impure coarse Si powder that was 700 MPa.

Figure 4. Uniform distribution of CNT in SiC matrix [29].

MATERIALS AND MANUFACTURING PROCESSES

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Figure 5. Open pores due to SiO and CO [30]. Figure 7. SEM images of polished surface: (a) RB-1 (b) RB-2, (c) RB-3 [36].

Barick et al. [36] used different particle size for the evaluation of their effect on reaction bonded boron carbide (RBBC). Residual silicon was present (Fig. 7) in the pores of RBBC ceramic that alternately affects the fracture toughness, and due to decrease in volume fraction of the weak interfacial area, the hardness of RBBC composite having large particle was more. The nanosized zirconia powder used by Li et al. [37] helps to reduce the porosity of composite by decreasing the quantity of residual silicon and inhibiting the growth of beta–silicon carbide, thereby increasing the flexure strength and fracture toughness corresponding to 37% and 36%, respectively. Li et al. [32] used V2O5 for lowering down the mullitization temperature and to promote densification rate. Effects of SiC rods used as reinforcement have been explored by Li et al. [38] in terms of nitridation, density, mechanical properties, and microstructure. Later on, Hu et al. [39] found that max 10% wt of SiC was sufficient to improve the mechanical properties of reaction bonded silicon nitride composite. Liu et al. [40] introduced two-step processing technique composed of freeze casting and carbothermal reduction reaction for the fabrication of Si3N4 bonded porous SiC ceramic to produce high strength and nearly net shape material.

Figure 6. Relation between flexure strength and sintering temperature [30].

The application area of CVD is generally associated with the deposition of a variety of materials such as metals, ceramics, and semiconductors as a thin or thick coating. A new technique called thermal plasma chemical vapor deposition (TPCVD) provides better results in the deposition rate as compared to conventional CVD and also the better control for the formation of a/b phase of both SiC and Si3N4 [41]. Hirai and Sasaki et al. [42] reviewed the applications, structure, and characteristics of CVD SiC. Foreign particles were missing from the grain boundaries (Fig. 8), and SiC processed by CVD has numerous applications in the areas of heat and corrosion resistant coating, fiber-reinforced composite materials, gas turbines materials, mirrors, semiconductors, fine powders, etc.

Results and Discussion The study of Ramavath et al. [43] revealed that the post CVD heat treatment proved to be better for increasing the grain size. Hirai et al. [44] used CVD for the fabrication of amorphous Si3N4– BN composite by varying the content of boron and also deduce that due to increase in boron content, the percentage of silicon content considerably decreased. Input gas ratio, surface kinetics, and deposition mechanism were mainly responsible for the growth rate of SiC film. The deposition mechanism changed from surface kinetics to mass transfer

Figure 8. Grain boundary showing no foreign particles [42].

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when lower gas ratio was used [45]. Yarbrough and Messier [46] in their study reported the current issues and problems that arise during CVD of diamond. Kang et al. [47] presented the effect of CVD parameters on the properties of M2 tool steel and Si3N4–TiC ceramic composite tool. As the deposition temperature increases, the microhardness of tool decreases due to the change in the microstructure from fine grained fibrous structure to columnar structure with coarse grains. CVD increased the life of TiC coated Si3N4–TiC tool by 8–10 times as compared to bare Si3N4–TiC. The mechanical properties of mullite fiber reinforced mullite matrix composites can be increased by depositing a single layer CVD SiC interphase. The improvement in the interfacial bonding of composite having SiC interphase has been reported with the increase in oxidation time [48]. Qiangang et al. [49] determined the microstructure and growth mechanism of SiC whiskers on carbon-carbon composites, and they found that b-SiC were uniformly deposited over the surface, and as the pressure ratio of H2/MTS increased, the purity of whiskers also increased and SiC whiskers were grown by vapor-solid growth process. Yang et al. [50] observed that the increase in the annealing temperature results in decreased flexure strength due to the oxidation of coating by oxygen or vapors and also ratio of weight loss has been increased. Naslin et al. [51] synthesized ceramic matrix composites by using a new processing technique known as pressure pulsed chemical vapor infiltration (P-CVI) and found it as a useful method for fabricating self-healing matrices. Park et al. [52] fabricated SiCf/SiC composite by whiskering process and concluded that the density of the composite can be enhanced by using cyclic whiskering process. CVI incorporation with tape casting process leads remarkable improvement in fracture toughness, flexure strength, and tensile strength due to minimization of whisker damage and bonding between interfacial and inter-laminar grains [53]. In a recent study, Zhou et al. [54] determined the effect of dip coated BN interphases on the mechanical properties of SiCf/SiC composites. Improvements in the fracture toughness and flexure strength were recorded for dip coated SiCf/SiC

Figure 9. Shows fiber pull out [54].

composites. Fiber pullout and crack deflection were the main reasons for better mechanical properties (Fig. 9). From the work done by Han et al. [55], it has been concluded that to obtain proper strength and interface bonding between fiber reinforced SiN matrix composites, low temperature CVI was best suited. Sol-gel method is generally a low-temperature synthesis process for the fabrication of oxide ceramics such as alumina, silica, zirconia, and titania in dense form and is used to produce oxide-oxide ceramic matrix composite with near net shape [56,57]. Naskar et al. [58] fabricated the fiber reinforced ceramic matrix composite by varying the process parameters of sol-gel method. Higher viscosities of sol have bad wetting properties. Multiple infiltrations help to improve green strength, machinibility, and flexure strength of material. Three-point bend test of ceramic matrix composites confirms the pseudo ductile character. Liang et al. [59], during their experimental investigation, found that the ZrC-SiC composite particle size was smaller, and it varies with the percentage of SiC particles. The presence of silica hinders the growth of zirconium particles. Zhang et al. [60] uses acetic acid as a chemical modifier for the fabrication of ZrB2–SiC composite powders. From EDS analysis, it was found that both ZrB2 and SiC co-exist in each particle and the particles have high chemical and phase homogeneity. Zou et al. [61] prepared a porous silicon nitride/silicon oxide composite by sol-gel route. The porosity of the composite keeps on decreasing as the content of SiO2 increased. The mechanical properties such as flexure strength, fracture toughness, density, and thermal shock resistance were largely improved. The dielectric constant of composite also increased with respect to increase in SiO2 content. Almeida et al. [62] tried to enhance the properties of ZrO2–MWCNT nanocomposites fabricated by sol-gel route and high pressure. The hardness and fracture toughness of nanocomposites increased about 15% and 110%, respectively, as compared to pure zirconia with a MWCNT content of 3 wt%. Chatzistavrou et al. [63] used sol-gel method for the fabrication of glass-ceramics and composites for their different dental applications. Inbaraj et al. [64] investigated the properties of alumina-MWCNT composite fabricated at different temperature range. The hot isostatic pressing after sintering helped in the improvement of mechanical properties and theoretical density of composite. The main drawback of using sol-gel as a fabrication technique is the occurrence of cracks in matrix during drying process [57,58]. Powder metallurgy is a precise manufacturing technology in which parts are produced from ferrous and non-ferrous materials. Processing steps of powder processing include mixing or blending of powders with suitable additives and lubricants, compaction of mixture followed by heating the compacts in the furnace under controlled atmosphere [22]. As reported by Cho et al. [11] in their valuable review, this technique can be applied to a number of composites. Slip casting involves the incorporation of reinforcement into the matrix melt and allowing the mixture to solidify. Irrespective to their specific gravities, slip casting is a suitable technique for fabricating ceramic composites [65]. Similarly, centrifugal slip casting technique is a suitable method that allows graded distribution of metal particles into the matrix phase [66].

MATERIALS AND MANUFACTURING PROCESSES

Kumar and Bhargava [67] used slip casting method for the uniform distribution of carbon black into ceramic composite in order to find minimum percentage of it to yield better mechanical and electrical properties. In 1988, Bhaduri et al. [68] confers that the boundary diffusion was a major reason for 100% dense Al2O3-ZrO2 composite during hot isostatic pressing. Noviyanto et al. [69] compared two different processing routes for SiCf/SiC composite and found that higher densification can be achieved in case of spark plasma sintering with less percentage of sintering additives, and it has been confirmed from the fractured surfaces also (Fig. 10). Verma and Kumar [221] used conventional sintering method for the fabrication of alumina composite and found that fracture toughness, hardness, and densification were highest at a temperature of 1700°C. Similarly, Lim et al. [8] performed the comparative study between two different fabrication routes used for aluminaMWCNT composite. Form the study, the authors found that tape casting followed by lamination and hot pressing was a better method to produce an alumina-MWCNT composite with excellent wear and mechanical properties. Dong et al. [70] used Y2O3 and CeO2 as sintering additives that restrict decarburization phenomenon during hot pressing, which leads to grain refinement (Fig. 11) and particulate homogeneity. Different carbon additives (CNT, carbon black, graphene, carbon black, carbon fiber) as a reinforcement in ceramic composite were used to improve the different properties of ceramic composite fabricated by hot pressing such as wear properties, electrical properties, and mechanical properties for composite [71–73]. The same has been proved by An et al. [74] by the evaluation of tribological properties of alumina-CNT composite. The findings of Balazsi et al. [75] confirmed that the carbon fibers deteriorate while sintering temperature increases, as shown in Fig. 12. The MWCNT as reinforcement is better than the other carbon reinforced specimen in terms of mechanical properties, density, etc. Post heat treatment of SiC-fiber-reinforced-glass-ceramic composite decreased the flexural strength by the formation of silica rich bridges and removal of carbon from the interfacial layer [76]. Shi et al. [77], based on their experimental

Figure 10. SEM Images of fractured surface (a) sample fabricated by HP with 12 wt% sintering additives (b) sample fabricated by SPS with 3 wt% sintering additives (c) sample fabricated by SPS with 5 wt% sintering additives [69].

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Figure 11. FE-SEM micrographs (a) Without sintering additives (b) With sintering Additives 0.1 wt% of Y2O3(c) With sintering Additives 0.1 wt% of CeO2 [70].

investigation, concluded that increase in hot pressing temperature and SiC grains leads to grain refinement, which was the main cause for decreased fracture toughness. The hardness and flexure strength improved with respect to increase in temperature and SiC content. Shimoda et al. [78] studied the effect of carbon nanofibers (CNFs) in SiC matrix fabricated by hot pressing via a transient eutectic phase route at a temperature of 1900°C. Agglomerates of CNT were found (Fig. 13) when percentage increased up to 10 wt%. In contrast to spark plasma sintering, hot pressing and hot isostatic pressing involve high temperature and longer duration that leads to the deterioration of reinforcement or decrease in reinforcement effect without producing a compact of desired properties [79]. Cha et al. [80] used a novel technique for the homogeneous distribution of CNT in alumina composite that consists of molecular-level mixing and an in-situ spark plasma sintering method. The mechanical properties of the composite were enhanced in terms of its hardness and fracture toughness due to load transfer mechanism and bridging mechanism, respectively, as shown in Fig. 14. The effect of different sintering temperature used in spark plasma sintering for the fabrication of alumina–SWCNT composite was determined by Jiang et al. [81] in terms of its mechanical and microstructural properties. The authors found that as the temperature increased from 1150°C, the CNT starts disappearing and relatively increases in graphite content. The Raman spectra also support the decrease in mechanical properties and mismatch of thermal expansion coefficient with

Figure 12. Showing the deterioration of CNT (a) As received Carbon Fiber (b) Carbon Fiber in Si3N4 Crystals (c) Damaged Carbon Fiber [75].

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Figure 13. Agglomerates of CNF in SiC matrix [78].

increasing temperature. The same has been deduced by Lin et al. in terms of its mechanical properties [82] while fabricating ZrB2-CNT composite within a temperature range of 1600°C–1800°C. Li et al. [83], on the other hand, used a combination of different sintering additives for lowering down the spark plasma sintering temperature of Si3N4 and found that MgO with AlPO4 was effective by lowering the temperature from 1400°C to 1200°C. Kumari et al. [84] correlate the spark plasma sintering temperature with thermal diffusivity of Al2O3–CNT nanocomposites and found that at a temperature of 1550°C, the composite have maximum thermal diffusivity and thermal conductivity as compared to pure alumina, but the heat capacity was highest at a temperature of 1450°C. Inam et al. [85] fabricated the alumina–carbon nanocomposites to find the relation between electrical conductivity with respect to spark plasma sintering temperature and percentage of carbon nanocomposites. As the temperature and percentage of CNTs increased, the grain size also increased, which decreased the grain boundary area and more conductive paths have been formed that leads to increase in electrical conductivity. Spark plasma sintering processing time is smaller, which saved more number of CNTs from damage, but the results of Raman spectroscopy indicate that the survival of CNTs at high temperature is very less [81]. Similar studies were made by Sarkar and Das [86] related to alumina-MWCNT nanocomposites, and they found that as the temperature increases, the well-developed equiaxed grains with reducing size were formed. At a temperature of 1700°C, relative density of more than 98% has been achieved. Due to high volume of MWCNTs

Figure 14. Bridging mechanism between CNT and Al2O3[80].

and with increasing temperature, the hardness of the nanocomposites decreases due to porosity and weak interface. From the experimental study of Niu et al. [87], it was found that the TiC as a sintering additive not only improve relative densities but also mechanical properties of ZrC ceramics with respect to spark plasma sintering temperature. Akin [222], in his experimental study, performed cell viability test on zirconia toughened alumina composite fabricated by spark plasma sintering and found no cytotoxicity to human osteoblast cells after 24 h of incubation. Spark plasma sintering technique helps to produce a dense, fine grained, and high strength ceramics and its composites [88]. Increase in temperature during spark plasma sintering helps to promote densification irrespective to reinforcement type. The pores gradually close with increase in temperature [89]. Using microwave energy for the sintering of powder material has an edge over the conventional sintering in terms of its reduced cycle time, reduced manufacturing cost, improved mechanical properties with retained fine and dense microstructure, environment friendliness, rapid heating rate, and better overall performance [79,90,91]. Different researchers [90–94] working in the field of microwave energy concluded that densification properties of material can be increased in a short duration by using this processing technique. The effect of soaking period during microwave sintering for WC-8Co was negligible, and at a temperature of 1450°C, the specimen was fully dense with more uniform WC grains [95], whereas the addition of Y2O3 in the mullite during microwave sintering has strong influence on the densification process and it promotes rapid sintering of mullite with shorter processing time [96]. Due to the collaborative effect of nanometer Al2O3 and microwave sintering, the microstructure and mechanical properties of alumina ceramics improved drastically [92]. The grain growth and its distribution were more uniform (Fig. 15) as compared to conventional sintering [97]. Different sintering atmospheres were used for the fabrication of WC-8 wt% Co during microwave sintering. A layer structure (Fig. 16) has been formed during N2 and N2 þH2 atmosphere, but no layers were observed during Ar atmosphere [98]. Benavente et al. [99] reinforced the alumina matrix with 5, 10, and 15 vol% of ZrO2 and sintered in monomode microwave furnace at 2.45 GHz in air at different temperature range of 1200°C–1400°C. Experimental data showed that densification as a function of sintering temperature and highest value for fracture toughness was obtained with 15 vol% of ZrO2. Hardness and young’s modulus value was highest with 10 vol% of ZrO2. Wang et al. [100] reported a novel technique for the fabrication of SiC-SWCNT composite. They formed ceramic directly on to the SWCNT rather than using powder metallurgical methods or CVD methods. Aggarwal [101] reviewed about the potential, significant development and future insight of microwave energy used as a sintering technique for different ceramics, composites, metallic materials, and melting of glasses. During the experimental study of Tianben and Hongzhi [102], they found that

MATERIALS AND MANUFACTURING PROCESSES

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Figure 15. SEM images of alumina ceramic with (a) Conventional sintering (b) Microwave sintering [97].

microwave sintering method inhibits the abnormal grain growth [103], and porosity of the cermets also gets decreased. Bykov et al. [104] studied the effect of heat produced during microwave sintering on the mass transport and phase transformation phenomenon for nanostructured materials. Experimental data showed that the phase transformation temperature has been decreased during microwave heating and its phase transformation rate depends upon microwave intensity. Due to decreased activation energy, mass transfer along the grain boundaries increased in microwave heating. Cheng et al. [105] fabricated Al2O3-TiC micro composite ceramic tool material through microwave sintering route. The relative density and hardness for that tool material keeps on increasing with increasing temperature as shown in Figs. 17 and 18, respectively. Similar study was done by Yin et al. [106] to explore the possibility of microwave sintering in the manufacturing of Al2O3/Ti (C,N) micro nano-composite ceramic tool with different holding time. Wang et al. [107] used microwave sintering to produce ZrB2- SiC ultra high temperature ceramic with excellent microstructural and mechanical properties. An

experimental study of Yang et al. [108] showed that for same PIP sintering temperature, microwave sintered SiC/SiC composite have better flexure strength as compared to conventional sintering. Kumar et al. [109] confirmed the improvement in properties of piezoelectric material sintered by microwave energy. In a recent study, Pian et al. [110] found three significant factors (hot spot effect, plasma behavior, and high frequency electromagnetic fields) that enhance the mullite formation during microwave energy. Xu et al. [111], in their experimental investigation, used a new technique known as microwave hot pressing to fabricate tungsten-copper alloys and found uniform and dense microstructure as shown in Fig. 19(a) and (b). Table 1 shows the overview of different fabrication methods employed in powder metallurgy processing technique, and Table 2 shows the comparison of different fabrication employed for CMCs. Figure 20 shows the contribution of different processing methods since the last 10 years. The fabrications of ceramic and its composite by using different sintering methods are not able to produce the accurate complex shapes due to shrinkage and other fabrication

Figure 16. SEM micrograph of WC-8 wt% Co processed under different environments (a) pure N2(b) pure Ar (c) N2 þ CH4(d) N2 þ H2[98].

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Figure 17. Relation between relative density and temperature [105].

Figure 18. Relation between Vickers hardness and temperature [105].

method limitations [9,113]. Machining includes the removal of material from the work piece generally in form of chips or abrasives. Material can be removed by two different methods that include conventional machining and modern machining methods. In conventional machining, the material is removed through the shear deformation such as turning in lathe, and also there must be a contact in between tool and workpiece whereas in modern machining material is removed by the action of different energies such as mechanical abrasion in

case of Ultrasonic machining (USM), thermo-electric energy in laser, plasma arc machining, etc. and there is no contact between tool and workpiece. Pawar et al. [114], in their review, presented the scenario of different machining process available for alumina ceramic and its composite, which is shown in Fig. 21. Grinding and drilling are the most common conventional machining methods used for ceramic and its composites. Electrolytic In Process Dressing (ELID) is a novel grinding technique used for surface grinding of ceramic composites. Its principle is based on electrochemical grinding [115]. In the past, Huang [114] studied the effect of high-speed deep grinding on the machining characteristics and surface integrity of yttria stabilized tetragonal zirconia. As the depth of cut increased, the surface roughness of the specimen gets decreased. The hardness of machined specimen also decreased due to the presence of microcracks. Normalized wear rate of wheel increased with respect to the increase in wheel speed and for all depths of cut. A similar study has been made by Huang and Liu [116] on the different advanced ceramic materials using high-speed deep grinding. Experimental data showed that material removal mechanism in Al2O3 and Al2O3-TiO2 was due to grain dislodgement or lateral cracking along the grain boundaries [120] whereas both ductile cutting and brittle fracture were found in yttria partially stabilized tetragonal zirconia (Fig. 22). An impregnated diamond bits has been used by Chao and Juntang [117] to make a drill in Al2O3 ceramic armor. The drilling efficiency was greatly affected by wall thickness, grain size, diamond concentration, and number of slots. Brittle fracture was a prominent factor as compared to ductile fracture in terms of material removal phenomenon. Intermittent grinding helps to dissipate heat produced during the machining process to the environment and thus lower the temperature of tool and workpiece. Reduction in normal and tangential force as well as high G-ratio can be obtained during intermittent grinding [118]. Cryogenic environment for grinding ceramic composite helps to reduce grinding forces, grinding energy, and surface roughness as compared to dry grinding. The lubrication properties of nitrogen jets and effective control of temperature help to reduce the surface damages [119]. Koshy et al. [121] used diamond grinding in conjunction with electrical spark for cemented carbides. Grinding performance has been enhanced and increased wear resistance at low discharge

Figure 19. (a) and (b): SEM micrographs of CuW80 alloys processed by Microwave hot press sintering [111].

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CNF

SWCNT, MWCNT, exfoliated graphite, carbon black

CVD-CNT, 1.8 vol%

SWCNT 10 vol%

CNT, 7.39 and 19.10 wt%

MWCNT (2–5 wt%) Carbon black (2 wt%)

MWCNT (2.4 vol%)

TiC (10 wt%)

Alumina

b-SiC nano-sized powder

Si3N4

Al2O3

γ- Al2O3

Al2O3

Al2O3

Reactive Al2O3

a-Al2O3

Alumina

Alumina

Alumina

3

4

5

6

7

8

9

10

11

12

13

14

15

Ultrasonication and ball milling in ethanol

Nano-SiC whiskers (btype) upto 30 vol%

TiC (30 wt%), Mo and Ni each 3 wt%, Y2O3 and MgO 4 wt% (as a sintering additives) MWCNT (1–5 vol%)

ZrB2

a-Al2O3

Alumina

16

17

Ultrasonic agitation, ball milling in water

Planetry ball mill with anhydrous ethanol

Traditional Milling

Ball milling

Ball milling

Ultrasonication, hand mixing and ball milling in dimethylforamide (DMF) Ultrasonication and magnetic stirring in isopropyl alchol and deionized water Planetry ball mill

High energy ball milling with zirconia and ethanol CVD Synthesis at 750°C

Molecular-level mixing

Ball milling with isopropyl alchol and sintering additives Al2O3 and Y2O3 Attritor mill with ethanol and sintering additives- AlN, Y2O3, Al2O3 followed by ultrasonication

Ball milling with anhydrous alchol

Planetry ball mill with ethanol

Mixing method used Ball milling

Zirconia (5–15 vol%)

Titania (13 wt%)

Zirconia (10 wt%)

MWCNT, carbon black, graphite and carbon fibers SiC

Si3N4

Reinforcement

2

CNT

Matrix Alumina

Dry pressing at 19 MPa followed by Cold isostatic pressing at 310 MPa, pressure less sintering (1500°C–1600°C), Ar gas

Microwave sintering (1150°C–1550°C), Conventional sintering (1150°C–1450°C) Uniaxially pressed at 60 MPa followed by Cold isostatic pressure of 280 MPa, Microwave sintering at 1250°C, Pressure less sintering (1250°C–1450°C) Cold isostatic pressing at 220 MPa, Microwave sintering ¼ 1200°C–1400°C uniaxial pressing at 30 MPa, cold isostatic pressing at 300 MPa, Microwave sintering 1850°C, Argon gas Uniaxially pressed (180–200 MPa), Microwave sintering (1500°C–1800°C), Ar gas

SPS (1100°C–1500°C), Load - 60 MPa

Cold isostatic pressing follwed by Pressure less sintering at 1500°C–1700°C

SPS, vacuum, 1000°C–1800°C

1. Dry pressing at 220 MPa 2. Hot Isostatic pressing at 1700°C, 20 MPa for 3 hr under nitrogen 3. Spark Plasma Sintering between 1000°C and 1650°C for 3–5 min under uniaxial pressure of 50 or 100 MPa Spark plasma sintering, vacuum pressure of 1 Pa Spark plasma sintering, vacuum pressure, 1150°C–1550°C SPS, vacuum, 1150°C, 1450°C, 1550°C

Hot pressed, argon atmosphere, 1900°C, 20 MPa

Sintering condition Hot pressing in argon atmosphere, 1800° C, 40 MPa duration 1 hr Dry pressed at 220 MPa, two step Hot isostatic pressing at temp 1700°C, 2 MPa for 1hour, 20 MPa for 3 hr Hot pressed between 1635°C and 1735°C under vacuum at 25 MPa

Overview of different powder metallurgy methods employed for the fabrication of ceramic matrix composites.

S.No. 1

Table 1.

Electrical resistivity ¼ 1.2 � 10,000 ohm cm, young’s modulus ¼ 428 � 14 GPa, HV ¼ 22.3 Gpa, Flexure strength ¼ 543 � 37 MPa, toughness ¼ 4.1 � 0.6 MPa m1/2

2014 [98]

Relative density ¼ 99%, HV ¼ 20 GPa, Young’s Modulus ¼ 367 GPa Flexure strength ¼ (625 � 78) MPa, Fracture toughness ¼ (7.18 � 1.21) MPa m1/2, Relative density ¼ 100% Relative density ¼ 99%, HV ¼ 21.2 � 0.26 GPa, Fracture toughness ¼ 5.18 � 0.1 MPa m1/2

(Continued)

2010 [112]

2014 [105]

2010 [107]

2012 [97]

1999 [93]

2015 [89]

2014 [86]

2010 [85]

2008 [84]

2007 [81]

2005 [80]

2011 [72]

2010 [78]

2010 [77]

2003 [75]

Year and Ref. No 2003 [74]

Relative density 98%, Flexure strength ¼ 576 MPa, HV ¼ 17.5 � 0.6, Fracture toughness ¼ 4.11 � 0.36

Relative density ¼ 99.50%, HV1 ¼ 19.95 � 1.11 GPa, Kic ¼ 4.83 MPa m1/2, r fs ¼ 260 MPa HV- 1892 GPa, young’s modulus ¼ 628 GPa Relative density ¼ 96%,

HV ¼ 1700 kg/mm2 (approx), K I.C./K I.C. alumina ¼ 1.3 MPa M1/2 Relative density ¼ 100%, HV ¼ 1610 kg/ mm2, KIC ¼ 9.7 MPa m1/2 Thermal diffusivity ¼ 13.98 mm/s2, heat capacity ¼ 3.76 J/g K, thermal conductivity ¼ 90.4 W/Mk Theroretical density ¼ 99%, Electrical conductivity ¼ 576 S/m

Results HV ¼ 2000 kg/mm2, friction co-efficient0.3 Improved mechanical properties from 15% to 37% using CNT as a reinforcement HV ¼ 25.2 � 1.0 GPa, Flexure strngth ¼ 615 MPa, Fracture toughness ¼ 7.6 � 0.2 Relative density ¼ 99%, HV ¼ 27 Gpa, Fracture toughness ¼ 5.7 MPa m1/2, Thermal conductivity ¼ 80 W/mK 1. For HIP Sample HV ¼ 15.17 GPa, Kic ¼ 4.84 MPa M1/2, Youngs modulus ¼ 261.33 GPa For SPS Sample Hv ¼ 18.73 GPa, Kic ¼ 4.41 MPa M1/2, Youngs Modulus ¼ 297.1 GPa

2015 [180]

2015 [15]

2015 [108]

Spark Plasma Sintering, 1250°C, Vacuum enviornment Ultrasonication, magnetic stirring in polyethylene glycol a-Al2O3 21

MWCNT (upto 5.7 vol%)

Ti 20

MWCNT (0.2–1 wt%)

Solution ball milling

Spark Plasma Sintering, 550°C, Vacuum enviornment

Flexure strength ¼ 551 � 51.7 MPa, Fracture toughness ¼ 29.8 � 2.8 MPa m1/2 Relative density ¼ 99.69%, compressive strength ¼ 1106 MPa, HV ∼ 2.3 Gpa, Electrical conductivity ¼ 2.75 MS/m, thermal diffusivity ∼15 mm2/s Fracture toughness ¼ 6.03 � 0.45 MPa m1/2, HV ¼ 19.18 � 0.33 GPa PIP process SiC 19

SiC fibers

Matrix Alumina S.No. 18

Continued.

Reinforcement MWCNT (0.15–2.4 vol%)

Mixing method used Ultrasonication, magnetic stirring in isoprpyl alchol

Sintering condition Cold isostatic pressing at 150 MPa, pressure less sintering (1500°C–1700°C), Argon Microwave sintering (800°C–1200°C), Conventional Sintering (800°C–1200°C)

Results HV1 ∼ 21 Gpa, Flexure strength ∼265 MPa, Fracture toughness ∼5 MPa m1/2

Year and Ref. No 2012 [215]

M. S. RAYAT ET AL.

Table1.

10

energy due to increase in fracture toughness of material. Material removal mechanism, surface roughness, and normal grinding forces generally depend upon the type of ceramic material used for high-speed grinding [118]. Comparative study between high-speed and conventional grinding has been performed by Yin et al. [122] for alumina and alumina-titania material. The material removal mechanism and surface integrity for both the materials were same. There was considerable decrease in the specific normal forces, tangential forces, and their force ratios when compared to conventional grinding. For grinding ceramic matrix composites, the diamond tool should have GVD binder, coarse grain size, hard diamond grains, and high diamond concentration [123]. A new novel technique, known as ultrasonic assisted grinding (UAG) for ceramic composites, has been reported by Azarhoushang and Tawakoli [124] and Li et al. [125], which results in reduced grinding force and improvement in surface quality. UAG is an effective method to machine hard and brittle material with improved cutting efficiency by ultrasonic vibrations [126]. Ultrasonic machining (USM) is a modern machining method in which the material is removed by the mechanical abrasion of slurry (mixture of fluid and abrasive particles) with the workpiece under the influence of high frequency waves [16–18]. USM is a non-thermal process suitable for hard material having hardness more than 40 HRC and irrespective of material properties [16,17,127]. Ultrasonic machining has proven to be the most productive and flexible method to create complex structures in the hard and brittle materials [128]. In USM tougher materials give low production rate, high tool wear, and a low surface roughness, and the relationships are reversed in brittle materials [129]. The basic mechanism of material removal in ultrasonic machining for hard and brittle material was proposed by Lee and Chan [12]. From their model, it was found that initiation and propagation of median as well as lateral cracks significantly contribute in material removal process. But form the model proposed by Sanjay [130], it was found that material removal generally occurred due to the propagation and intersection of median and radial cracks. For micro brittle fracture to occur, shocking force must be greater than critical load and also increase in material removal rate and surface roughness depends on the amplitude of tool, static load on tool, and grit size of abrasive grains [12]. Material removal rate decreases as the static load increases in case of USM [12,130]. Surface integrity (Fig. 23) was good in case of ultrasonic machined surface within the used parameters. Flexure strength and Weibull modulus can also be improved by decreasing the grit size of abrasive grains [9]. Rajurkar et al. [131] performed the experimental study to find the mechanics of ultrasonic machining of alumina in terms of its material removal rate. The dynamic test indicates that the lower velocity impacts removed the material due to structural disintegration and particle dislocations whereas high velocity impacts removed the material through a network of intergranular microcracks (Fig. 24). In 2001, Lee and Jianxin [132] reported the variation in the material removal rate, mechanical properties, and surface roughness of a composite with respect to whisker orientation. Material removal rate and surface roughness of specimen

MATERIALS AND MANUFACTURING PROCESSES

Table 2. S.No.

Comparison of different fabrication methods. Route Advantages . . . . . .

Near net shape Low processing time High densification properties Enhanced mechanical properties High deposition rate Applicability to number of materials

Sol-gel and Polymer Infiltration Pyrolysis

. . . . .

Flexibility in selecting fibers and matrices Complex shapes can be produced Uniformity and purity of matrix Better control over matrix composition Densification temperature is less

Powder Processing

.

Simple and complex shapes can be formed depending upon the sintering process High densification properties Good mechanical properties Rapid process

1.

Reaction Bonding

2.

Chemical Vapor Deposition

3.

Chemical Vapor Infiltration

4.

5.

. . .

decrease when h ¼ 00 to h ¼ 900 whereas the fracture toughness increased about 156%. Jianxin and Taichu [133] and Bertsche et al. [123] extend their support to Lee and Jianxin [132] observations. Bhosale et al. [2] optimized the process parameters of ultrasonic machining for aluminazirconia ceramic composite. They found that the amplitude has a significant effect on the tool wear rate, surface roughness, and material removal rate. An increase in slurry concentration from 10% to 40% results in increased tool wear rate and less material removal rate due to loss of energy. Various researchers [113,123,134,135] have performed rotary ultrasonic machining (RUM) and enlisted the advantages in terms of its improved material removal rate, reduced cutting forces, and decreased tool wear rate as compared to conventional machining process. Tool wear rate has been decreased by 36% compared to conventional grinding [123] as shown in Fig. 25. Lalchhuanvela et al. [136] have done the multivariate optimization of control factor in USM process. From their analysis, it was found that higher value of slurry concentration, slurry flow rate, and tool feed rate results in higher material removal rate whereas the value of surface roughness decreased. Majeed et al. [137] found that addition of LaPO4 up to 30%

Figure 20. Contribution of different processing methods in CMCs.

Disadvantages . .

Limited to simple shapes High porosity

Hazardous by-products produced during chemical reaction . Leads to stresses in films due to difference in thermal expansion co-efficient . Low processing time . High residual porosity . Low production rate . Long processing time . Multiple iterative processes to obtain dense structure . Low production rate . Crack formation in matrix during cooling phase . Uniform distribution of reinforcement into matrix phase is difficult task . High processing temperature . Chances of damage to reinforcement during compaction process .

11

Cost

Temperature

High

Up to 1700°C

High

Up to 1600°C

High

Up to 1800°C

Low

Up to 1400°C

Medium

Up to 1800°C

improved the machinibility of alumina in ultrasonic machining. The outflow of crushed chips, incoming slurry, and higher material removal rate take place more smoothly with tool having rectangular sectional profiles as compared to square sectional profiles [138]. Komaraiah et al. [139] analyzed the surface roughness and accuracy in USM for different ceramic materials by using different tool materials. Rotary mode of USM was found to be better in terms of higher MRR, lower surface roughness, machining axisymmetric holes, and better accuracy of the machined components when compared with conventional USM. In USM, hole integrity can be minimized by using a smaller grit size but cannot be eliminated [140]. A jet of fine grained abrasives mixed with air or some other carrier gas at high pressure is used for machining hard and brittle material [16–18]. In 1993, Ramachandran and Ramakrishnan [141] reviewed the possibilities of AJM to machine metals, nonmetals, and composite materials. AJM is a reliable machining method for the cost-effective fabrication

Figure 21. Contribution of different machining process for alumina and its composites [114].

12

M. S. RAYAT ET AL.

Figure 22. Material removal mechanism in (a) Al2O3(b) Al2O3-TiO2(c) Y-TZP [116].

of microdevices from hard and brittle materials [142], and polishing of surface, deburring, and finishing operations can be carried out effectively [16,18,141–145]. In AJM basically the material is removed due to the erosive wear, resulting from the crack propagation and intersection of cracks produced by impacting particles [16,143]. Wakuda et al. [146] tested the machinibility with respect to ceramic material (Fig. 26) in abrasive jet machining and recorded fracture toughness and hardness were the main parameters affecting material removal rate. Strength degradation did not take place for the AJM ceramic surfaces [146,147]. In 1996, Khodke et al. [148] proposed analytical model to find the material removal in AJM for brittle materials. They suggested that impact velocity, material properties, geometry, and abrasive particles properties affect the MRR significantly.

Figure 23. SEM images of USMed surface of ceramic composite [9].

Wakuda et al. [147] in another study compared two different micromachining methods. They found that the smooth curve was obtained in case of AJM as compared to laser beam machining (LBM) and the strength of laser machined surface also deteriorates. Abrasive water jet machining (AWJM) is a variant of water jet machining (WJM) in which abrasives are mixed with high velocity water jet for cutting action [16–18]. AWJM is thousand times more powerful than water jet [18]. AWJM is governed by main five parameters that are hydraulic parameters, abrasive parameters, mixing chamber, traverse parameters, and material properties [149]. Wada and Kumon [150] in 1993 investigated the effect of different angles of jet stream in AWJM and found 90° was the best angle of the abrasive nozzle to cut Si3N4 ceramics. Wang et al. [151] used multipass cutting technique to increase the cutting capability of AWJM for alumina ceramics in association with a jet forward angle of 80° [152]. In 1994, Hocheng and Chang [153] did the material removal analysis of ceramic plates by AWJM cutting and concluded that amount of material removal is primarily governed by two main factors, i.e., hydraulic pressure and abrasive flow rate. Traverse speed also affects the kerf width and taper ratio. Later on, Srinivasu et al. [154] studied the influence of kinematic parameters, i.e., jet impingent angle and jet feed rate on kerf geometry of silicon carbide ceramics in AWJM. Maniadaki et al. [155] proposed a multiparticle simulation method to evaluate the effect of impact angle and jet velocity in crater circularity. Surface roughness of ceramic materials in case of AWJM depends upon operational parameters such as water pressure, abrasive mass flow rate, nozzle stand-off distance, and traverse speed [156].

MATERIALS AND MANUFACTURING PROCESSES

13

Figure 24. Surface characteristics (a) Low velocity impact (b) High velocity impact [131].

Laser Beam Machining (LBM) is an alternative method for machining hard and brittle materials [16–18,145,157]. Samant and Dahotre [158] in their review classify the laser machining into three different categories: one-, two-, and threedimensional machining. Laser drilling, laser cutting, and laser milling and turning are the examples of one-, two-, and threedimensional laser machining, respectively. The experimental technique fracture – machining element used by Tsai and Chen [159] for laser milling of cavity in ceramic material is not only suitable for edge milling but can also be employed for center milling with less power requirement. Chang and Kuo [157] reported 22% and 20% reduction of feed force and thrust force, respectively, in laser assisted machining (LAM) of Al2O3 ceramics. Surface produced during LAM was smooth and straight due to the plastic flow of material whereas in conventional planning, surface was rough as shown in Fig. 27(a) and (b). To enhance the machinibility of the work surface, an intense energy of laser beam was used thus affecting the deformation behavior [160]. Machinibility of materials in LAM can also be increased by reducing the mechanical strength [161]. Dubey and Yadava [162] reviewed and explored the possibilities of LBM for a wide range of materials. Laser beam machining for thick materials and microparts requires considerable research work. LBM performance depends upon laser parameters, material parameters, and process parameters [163]. Kim et al. [164] machined CNT/Fe/Al2O3 nanocomposites by laser micromachining and reported that nanocomposites with higher CNT content produced good machining result and no microstructural damage (Fig. 28) was detected by laser energy.

Figure 25. Comparison between RUSM and conventional grinding [123].

Laser cutting can be used to produce cantilever structures from silicon nitride based composite materials. However, there is significant difference in the cutting quality in the laser entrance as compared to exit surfaces [165]. Sola and Pena [166] investigated the effect of different machining conditions on the Al2O3-ZrO2 composite by near infrared ray (NIR) pulsed laser. Different factors that affect the laser interaction process were substrate temperature, the plasma shield, the pulse number, the sample position with respect to the focal plane, and the working frequency. Yan et al. [167] mentioned the benefits of underwater machining using CO2 laser for deep cavities in alumina. Underwater machining using CO2 helps in reducing the substrate defects like recast layer, dross, heat damages, and cracking. Venkatesan et al. [168] in concluding remarks reviewed the future possibility of different other machining processes such as milling, grinding, and drilling by using laser technology as past research basically focused on laser assisted turning only. Hanon et al. [169] performed the drilling of alumina ceramic using Nd:YAG laser and found that for a single laser pulse duration crater depth can be controlled as a function of peak power and pulse duration. The multi-objective optimization approach of Bharatish et al. [170] proposed that to get nominal entrance circularity, exit circularity, minimum HAZ, and minimum taper, the laser parameters such as high frequency, high laser power, moderate scanning speed, and lower hole diameter are preferred. Samant and Dahotre [171] did significant research in the field of three-dimensional laser machining of structural ceramics and found that material removal increased with increase in heating rate. Electric discharge machining is one of the modern machining methods used as a precision machining method in which the electrically conductive hard material is removed by thermoelectric phenomenon between workpiece and tool electrode. In EDM generally a spark is generated between the inter electrode gap to remove the material from workpiece. Due to noncontact machining method, it has been widely used for tool and die making processes, parts for aerospace, automotive industry, and surgical components [17,18,145,172–174]. Lauwers et al. [175] did the detailed investigation on the material removal mechanism of electrical conductive ceramics and found that in addition to melting/evaporation and spalling [176–180], other material removal mechanism such as oxidation and dissolution of base metal may also take place.

14

M. S. RAYAT ET AL.

Figure 26. Impression of AJM face for different material [146].

Different researchers such as Abbas et al. [173], Shrivastva and Dubey [181], and Jain [145] discussed the current developments in EDM carried out by researchers based on the trend and their interest, for example: Ultrasonic Vibration EDM Micro EDM Dry EDM Powder-based EDM Water EDM Similarly, in their research, Deng and Lee [182] used ultrasonic and abrasive blast surface finishing method in order to improve the surface integrity of ceramic composites and found that both procedures were effective to improve the surface quality of wire EDM surface (Fig. 29) In 1995, Zhixin et al. [183] developed a new machining technique for making holes in engineering ceramics with high material removal rate named as ultrasonic vibration pulse electro-discharge machining (UVPEDM). Egashira and Masuzawa [184] reported high tool wear rate (TWR) when ultrasonic vibration has been given to workpiece instead of tool. In another study, Yu et al. [185] claimed the increase in productivity of EDM by using ultrasonic vibrations as compared to conventional EDM due to improved flushing

and machining conditions. Table 3 shows the percentage increase in the MRR for normal EDM and USEDM. In a recent research done by Mohammadi et al. [186], they developed a new method for turning with a combination of WEDM and ultrasonic vibration. They found that ultrasonic vibration helps to reduce the friction between wire and wire guide that results in higher MRR. By using UVPEDM, one can achieve double depth micro holes more easily as compared to normal EDM [187]. Wei et al. [188] confirmed that by using vibrating tool and dielectric deep flushing, the debris evacuation was enhanced that further results in better MRR and surface qualities as shown in Fig. 30. Liu et al. [189] discussed the process capabilities of microEDM, and by using this technique, they manufactured a twoand three-dimensional micro-parts such as micro- compressor and miniature gas turbine impeller (Fig. 31). In their experimental study, Liu and Huang [219] performed microelectric discharge machining of Si3N4 reinforced with different weight percentage of TiN. The particle sizes of TiN were found to be a promising factor in determining the strength, toughness, and electrical resistivity of the ceramic composite. Schubert et al. [220] in their review presented the current scenario and past investigation done on microelectric discharge machining for

Figure 27. (a) and (b) Micrograph of the surface during conventional machining and laser assisted machining [157].

MATERIALS AND MANUFACTURING PROCESSES

15

Figure 28. SEM micrograph of (a) Al2O3(b) 2 wt% CNT (c) 8 wt% CNT [164].

ceramics. They enlisted the various advantages of micro-EDM in terms of non-contact technology, close tolerances, low effect on material, etc. Increase in the discharge energy during micro-EDM leads to poor surface characteristics in terms of cracks and micropores [224].

The main aim of using dry EDM is generally used to reduce the pollution caused by different dielectric fluid that results in the formation of vapors during machining [190]. Tzeng and Lee [191] identified the main powder characteristics that can affect EDM performance as particle size, particle

Figure 29. SEM images of (a) Wire EDM surface (b) Abrasive blasting treated surface (c) Ultrasonic Vibration treated surface [182].

16

M. S. RAYAT ET AL.

Table 3. S. No.

Comparison of MRR (mm3/min) between EDM and USEDM [185]. MRR (EDM) MRR (USEDM) Increasing rate

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

17.5753 0.5148 4.7565 0.3642 2.6275 0.4276 13.8593 0.5523 6.1571 2.1879 7.5233 2.7199 4.6301 4.4373 4.4337

19.6969 1.1162 11.7247 0.7911 11.1238 0.5785 22.7486 1.0681 9.1977 2.5784 11.3854 7.5454 5.8410 5.3401 5.5321

12.07% 116.82% 146.50% 117.22% 323.36% 35.29% 64.14% 93.39% 49.38% 17.85% 51.33% 117.41% 26.15% 20.35% 24.77% Figure 31. Gas turbine impeller manufactured by Micro-EDM [189].

concentration, particle density, electrical resistivity, and thermal conductivity. Yu et al. [192] confirm from their experimental investigation that the dry EDM milling is a better technique as compared to oil EDM milling and oil die sinking EDM in terms of three-dimensional milling of cemented carbides. Figures 32 and 33 show the work removal rate and electrode wear ratio in groove machining, respectively. In a recent study, Dhakkar et al. [223] compared the different near dielectric mediums used in electric discharge machining. Glycerin-air mixture was found to be suitable in terms of high material removal rate, and also tool wear rate was decreased as compared to conventional EDM. The experimental investigation done by Jahan et al. [193] primarily focused on the improvement of surface properties of cemented tungsten carbide by using dielectric mixed with graphite nanopowder in sinking and milling micro EDM. Graphite nanopowder helps to improve the surface finish with uniform distribution of sparks in both sinking and milling micro EDM. In a recent research, Kolli and Kumar [194] optimized the concentration of surfactant and graphite

Figure 30. The effect of electrode vibration and dielectric deep flushing on surface quality: (a) Regular EDM (b) with tool vibration (c) with deep flushing [188].

powder in dielectric fluid to improve material removal rate and reduce surface roughness and less recast layer. Mai et al. [195] enlist the benefits of CNT mixed in dielectric fluid over machining time, electrode wear rate, and surface roughness. The machining efficiency and surface roughness were improved by 70% and 66%, respectively, in case of dielectric mixed with CNT in EDM. Figures 34–36 show the machining time, electrode wear rate, and surface roughness for different powders. Similar research has been done by Sari et al. [196] in order to find out the relevant machining performances using CNT mixed dielectric in EDM with different machining parameters. Dielectric mixed with CNT makes the EDM more efficient when the machining parameters are set at low pulse energy. Due to large heat absorption, the thickness of recast layer has been decreased. Table 4 shows the thickness of recast layer in EDM. Liew et al. [197] found the sticking of carbon nanofibers to the workpiece surface that alternately enhances the electric discharge machinibility. Atefi et al. [198] confirmed the improvement of machining performance in terms of surface roughness of AISI D2 steel in the presence of dielectric containing MWCNT in EDM. In the experimental study of Bai et al. [199], a new machining method of powder mixed near dry-EDM (PMND-EDM) has been used. Air pressure and tool rotational speed were the least significant parameters that affect MRR in PMND-EDM. For machining of ceramics by EDM, they must possess minimum electrical conductivity and homogenous structure irrespective of their hardness and toughness [200].

Figure 32. Work removal rate in case of Oil EDM and Dry EDM milling [192].

MATERIALS AND MANUFACTURING PROCESSES

17

Figure 35. Comparison of different powder concentration on TWR [195]. Figure 33. Electrode wear rate in case of Oil EDM and Dry EDM milling [192].

In 1996, Mohari et al. [201] used assisted electrode machining (AEM) method for insulating ceramics with the help of sinking EDM and wire EDM (WEDM). Similarly, Muttamara et al. [202] tried to find the possibility of micromachining of Si3N4 by assisting electrode machining in EDM with different assisting layers. AEM is a best suited method for machining three-dimensional complex shapes by WEDM [203]. Suitable machining conditions recommended by Banu et al. [204] for ZrO2 were: copper adhesive as assisting electrode, positive polarity for workpiece, one way circulation for dielectric, and feed rate of 3 µm/s. Liu and Huang [205] worked on the effect of EDM on strength and reliability of TiN/Si3N4 composites and showed that the flexure strength of EDM processed sample has been significantly low when compared with polished samples. A new technique proposed by Lauwers et al. [206] for machining ceramics having low conductivity by sinking EDM results in 50% time reduction. Bonny et al. [207] suggest that the secondary electroconductive phase in ZrO2 ceramic composites has significant effect on the frictional behavior, mechanical properties, and electrical discharge machinability, and the material removal mechanism for ZrO2–WC was purely melting and evaporation [208]. Chiang [209] from his experimental study recommended that discharge current and duty factor were the significant factors that affect MRR whereas for electrode wear rate and surface roughness, the influential parameters were discharge current and pulse on time. Lin et al. [210] suggested machining polarity and peak current were the significant parameters in order to improve MRR and electrode wear rate, and for surface roughness peak current was the most influential parameter.

Patel et al. [211] identified the main machining parameters involved in EDM as: pulse on time, duty cycle, discharge current, and gap voltage. Their results show that the pulse on time and duty cycle were the main parameters for improving the surface roughness. In another work, Zhang [179] found that the increase in content percentage of TiN and number of power transistors leads to higher material removal rate and high surface roughness values. Hanaoka et al. [212] compared the two different processes that are EDM using AEM and normal EDM for MRR, surface roughness and electrode wear ratio of Si3N4, Si3N4- CNT, and Si3N4- graphene nano platelet. By comparing the results of both processes, it was proved that AEM was a good method for obtaining better machining performance in EDM. Tak et al. [213] observed the poor dimensional accuracy and circularity of micro holes in Al2O3 – CNT composite having 10 vol% of CNT as compared to 5 vol% of CNT due to tangles of CNT (Fig. 37). Similar study was done by Malek et al. [214] to find the effect of applied voltage on MRR, relative tool wear, and surface roughness for Si3N4 – CNT (5.3 vol%) composite. They found that the relative tool wear rate decreased about three times as compared to Si3N4-TiN (40 vol%) composite (Fig. 38). In a recent study of Melk et al. [216], it has been found that high temperature generation during electrical discharges was responsible for the damaged and disordered CNTs.

Figure 36. Comparison of different powder concentration on TWR [195].

Table 4. Thickness of recast layer with and without CNTs [196]. Thickness of white Without using With using Difference layer cast [µm] MWCNTs MWCNTs (%)

Figure 34. Comparison of different powder concentration on machining time [195].

Sample Sample Sample Sample Sample

1 2 3 4 5

9.4 7.33 11 5.20 7.59

6.05 5.07 6.95 3.95 5.53

35.64 30.83 36.82 24.04 27.14

18

M. S. RAYAT ET AL.

Figure 37. SEM images of microholes drilled by EDM (a) 5 vol% of CNT (b) 10 vol% of CNT [213].

Figure 38. Comparison of relative tool wear rate for Si3N4-MWCNT and Si3N4- TiN [214].

Conclusion As discussed in the literature review, some of the major key challenges that have been encountered by different researchers while fabrication processes are enlisted below: 1. The chemical methods used by various researchers [19–59] for the fabrication of ceramic and its composites are used to produce near net shape with low processing time and temperature but on the other hand the composite formed by reaction bonding method found to be more porous that leads to deterioration of mechanical properties. 2. In sol-gel method, formation of cracks in the matrix phase is a major problem during drying process [61,62]. Sol-gel and polymer processing results in low yield and large shrinkage volume and requires multiple steps to achieve densification [10]. 3. Uniform distribution of reinforcement is another major challenge encountered by various researchers. 4. Conventional sintering of refractory materials at high temperature with long soaking period leads to abnormal grain growth, poor mechanical properties, and deterioration of secondary phase [112]. 5. Hot pressing and spark plasma sintering methods are limited to only formation of simple geometrical shapes with limited size [100,112]. Challenges in the machining process: 1. As reported by Tawakoli and Azarhousang [13,124], most of the traditional machining process includes high

processing and finishing cost even up to 90% of manufacturing cost. Diamond grinding as conventional method for machining hard ceramics was incapable for producing components with complex geometric shapes [214]. 2. According to Pandey and Shan [16], material removal takes place due to generation of high frequency waves in ultrasonic machining but it is not able to cut the material at a faster rate and chances for holes to breakout at the bottom part. 3. Liu and Schubert [217] found that delamination and kerf deformation are produced on the substrates during abrasive water jet machining, and surface quality was not good as it is suitable for rough cut application [182]. 4. As EDM requires a conductive material for processing, some researchers [201–204,212] used assisting electrode method (AEM) for machining insulating ceramics. 5. While laser machining is used for ceramic and its composites, chances are there for the presence of heat affected zone similar to the case for electric discharge machining and also recast layer is formed [182,218]. Further improvements in the processing and machining techniques are required in order to develop a ceramic composite that can enhance its acceptability in different engineering applications. Fabrication methods such as Melt Infiltration in reaction bonding, Molecular level mixing, thermal plasma chemical vapor deposition (TPCVD), microwave processing, etc. yet to be explored. Out of these fabrications methods, processing with microwave energy provides some highlights such as rapid heating rate, almost 100% conversion of electromagnetic energy into heat energy, reduced processing time, and environmental friendliness. Microwave sintering has some limitations and complexity, yet these can be overcome by other advantages as compared to different processing techniques. As far as machining of CMCs is concerned, it is one of the difficult tasks to be performed due to its different physical properties. Numbers of conventional and modern machining methods are available, but each process discussed above in the review has its own limitations. Apart from these machining methods, EDM is the modern machining method that is primarily used for the material having good electrical conductivity and capable to machine hard and intricate parts. Researchers in the past have successfully performed the machining of ceramic with electric discharge machining or by assisted electrode discharge machining either by doping a reinforcement phase having good electrical conductivity or by making conductive layer.

MATERIALS AND MANUFACTURING PROCESSES

Further improvement and optimization of EDM parameters (electrical and non-electrical) for CMCs will bring about enhanced understanding of machining behavior related to CMCs materials and will also help in developing a mathematical model for different CMCs materials and machining parameters.

Acknowledgments The authors are grateful for the help provided by Probal Kumar Das (Retd. Scientist G, CGCRI, Kolkatta), Lovely Professional University, Phagwara, Almatis, ACC Ltd., India and Dessica Chemicals Ltd. This paper is the outcome of PhD work done by the first author.

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