Physicomechanical properties of ultrahigh temperature heteromodulus ceramics based on group 4 transition metal carbides I. L. Shabalin*1, Y. Wang1, A. V. Krynkin1, O. V. Umnova1, V. M. Vishnyakov2, L. I. Shabalin3 and V. K. Churkin4 Highly densified TiC, ZrC and HfC based ultrahigh temperature heteromodulus ceramics (HMC), containing 10–50 vol.-% of low modulus phase in the form of particulate graphite, were prepared by hot pressing at 2700uC and 12 MPa in argon atmosphere. The microstructure, elastic characteristics, flexural and compressive static strength, fracture toughness, impact resistance, hardness and thermal expansion were investigated and compared with those available in earlier works for clear understanding the composition–property correlations and anisotropy of this type of HMC composites. Different thermal shock resistant parameters for the HMC were calculated on the basis of obtained experimental data. A new principle of optimum materials design for the compositions in the refractory carbide–graphite systems is exemplified by the TiC–C HMC materials. Keywords: Carbides, Carbon, Composites, Mechanical properties, Heteromodulus ceramics
Introduction The discovery of ridge effect1–4 in the oxidation of carbide–carbon heteromodulus ceramics5,6 (HMC) put forward this type of ultrahigh temperature materials in the leading position for potential applications in the extreme environments, which are faced with exploitation of various thermal protection systems in aerospace technologies and nuclear engineering. Most modern advances in rocket motors and propulsion assemblies are connected with increase of intensive thermochemical erosion and thermomechanical loading effects on the surface of materials in the area of critical cross-section of nozzles. Further development of hypersonic vehicles with extremely high speed in the dense layers of atmosphere (antiballistic missiles) as well as design of re-entry space vehicles with reusable hot structures (various types of space shuttles) incorporates sharp surfaces, such as wing leading edges and/or slender fuselage noses, to increase the aerodynamic performance. These innovations require the novel materials with enhanced damage tolerance to high speed/high enthalpy and chemically active gaseous/plasma flows. The materials should be able to withstand tremendous 1
Institute for Materials Research, University of Salford, Greater Manchester M5 4WT, UK Dalton Research Institute, Manchester Metropolitan University, Manchester M1 5GD, UK 3 Russian State Professional and Pedagogical University, Yekaterinburg 620012, Russia 4 Samara State Aerospace University Togliatti Campus, Togliatti, Samara Area 445038, Russia 2
*Corresponding author, email
[email protected]
ß 2010 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 29 April 2009; accepted 21 September 2009 DOI 10.1179/174367509X12535211569431
thermal shocks/heating rates (up to 104 K s21) and intensive heat fluxes (.2 MW m22). Numerous attempts to cover the carbon–carbon composites (CCC), which are widely used in rocketry currently, by different coatings, showed the fruitlessness of this approach, because of the weak adherence, which is inherent to any kind of matter in contact with basic crystallographic planes (00l) of graphite-like phases. In contrast with various CCC, the HMC, treated by the ridge effect knowledge based technique, can be coated by the self-assembling (synergetic) protective scales functionally graded, which are perfectly adhered with the substrate due to formation of intermediate carbon doped oxide/oxycarbide layers1,3 and conserved continuous (throughout the scales as well as substrate) reinforcement.4 Furthermore, in this case, the protective scales are characterised by the highest melting (2700–2900uC) and boiling (5200–5400uC) points, which are known for the matters chemically stable in oxygen. The group 4 transition metal carbides are real champions among the variety of materials concerning their melting points (up to 3950uC for HfC) and hardness (up to 40 GPa for TiC), so because of these properties, they are candidate materials for many modern engineering applications.5 To avoid the dramatic effects caused by brittleness of the carbides is a key problem of materials engineering design for a long period of time. The earlier engineering solutions in this way, which were connected with development of ceramic–metal composites (cermets), cannot be useful in the range of temperatures higher than 2000uC. The generally imperfect impact tolerance and low thermal
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shock resistance, which are inherent to the transition metal carbides, can be improved noticeably by the addition of a low modulus phase such as graphite,6 and this is without considerable restrictions to the applications at ultrahigh temperatures, because the eutectic points in the TiC–C, ZrC–C and HfC–C systems are positioned close to 3000uC. The first studies of strength and elastic properties for the transition metal carbide–carbon HMC7–17 began simultaneously with the initial development of different manufacturing methods (hot pressing/sintering,7–9,15,17,18 arc casting,10–12,17 chemical vapour deposition13,16 and melt/chemical vapour impregnation/infiltration of carbon structures14,19) for preparation of these materials. The main goals of the measurements were to find out the general correlations between the employed technological parameters and some mechanical properties of the newly produced composites, and thus, by means of it to improve/optimise a manufacturing technique. That is why the researchers restricted themselves in determination and analysis of few characteristics for the studied HMC compositions. Nevertheless, the joint consideration of the results obtained in these papers and later works18–30 allowed understanding some basic trends in physicomechanical behaviour of the carbide–carbon HMC. These findings were connected mainly with steady decrease in strength and/or elastic characteristics with increasing carbon/graphite phase content7,11,16,17 and considerable anisotropy of these properties.7,9,17 Along with it, the experimental studies cleared up (sometimes without any explanation of the phenomenon) that the HMC microstructures prepared by different techniques possess the outstanding ability to absorb the elastic energy released during crack propagation because of the specific capability connected with crack blunting and/or diverting. The purpose of this work was to investigate the basic physicomechanical properties of the HMC based on group 4 transition metal carbides in general correlation with compositions and microstructures, estimate the thermal shock resistance parameters of the materials, which are mostly affected on the mechanical characteristics, and subsequently analyse/summarise the newly obtained experimental data together with those in the earlier published papers. To simplify the correlation models in the present study, the preference was given to the particulate carbide–graphite composites prepared by the common hot pressing method. This technique has allowed preparing various carbide–carbon HMC in a wide range of compositions at practically the same technological conditions.5 Taking into account the expected sensitivity of ridge effect behaviour to the materials composition, in particular to the value of Pilling– Bedworth ratio variation for HMC, which is in practice strongly dependent on the volume fraction of low modulus carbon phase,4,31 the focus of attention was produced first of all on property–composition correlations. The similar approach provides an opportunity in materials design of HMC to improve/achieve the necessary characteristics of a material by the certain optimisation/selection of its composition that is especially important in the case of subsequent transformations of the substrates into various functionally graded materials by means of ridge effect knowledge based technique.
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Experimental The particularities of synthesis, processing and characteristics of the fine powders of transition metal (titanium, zirconium and hafnium) carbides as well as the properties of the initial natural graphite powder, which were used as starting materials in high temperature hot pressing procedures, are given elsewhere.1,2,32,33 The blanks of transition metal carbide HMC with various volume contents of graphite were hot pressed unidirectionally at 2700uC under an applied pressure of 12 MPa in argon atmosphere. All details of the employed manufacturing method, including technological map, are given in the recent publication.5 The prepared samples of HMC materials were examined by means of chemical, atomic absorptive, X-ray diffraction (XRD), optical and scanning electron microscopy analyses as well as microanalysis by electron probe and energy dispersive X-ray spectroscopy. The compositions of the samples were identified by XRD phase analysis, and the lattice parameters of the phases were measured by means of XRD structural analysis employing a diffractometer DRON-4 unit (filtered Cu Ka radiation) with standard software. Scanning electron microscopy (SEM) images were obtained at accelerating voltage U512–25 kV employing both secondary electron (SE) and backscattered electron (BSE) detectors. The physicochemical characteristics of the studied materials and their constituents are reported in Tables 1 and 2. In this study, the mechanical properties of HMC samples were investigated at room temperature. The elastic characteristics of the materials were determined along perpendicular and parallel directions to the axis of hot pressing (Z) by the ultrasonic method employing a commercial installation. The flexural strength was measured by a three-point bending test with 30 mm span. The dimensions of the test bars, which were cut off from blanks in both perpendicular and parallel directions to the Z, were 868640 and 10610612 mm3 for flexural and compressive strength respectively. The flexural and compressive strength measurements were carried out on an Instron type machine with 0?25 mm min21 crosshead rate. Fracture toughness was estimated by a single edge vee notch beam method with 16 mm span, 0?05 mm min21 crosshead rate and 365625 mm samples with notch of 2 mm in depth, which were cut off in perpendicular directions to the Z. Dynamic bending test was carried out on a pendulum impact testing machine MK-0?5-1 (distance to the impact point: 0?328 m, hammer weight: 3?94 N) using test bars with the dimensions (length: 55 mm, notch: 2 mm in depth and 2 mm in width, square of working cross-section: 1–1?2 cm2) accordingly to the GOST 9454 standard. To assess the acceptability of different standard methods, hardness of the materials was measured by indentation with a four-facet diamond pyramid (GOST 2999 standard), 120u diamond cone and/or 1/16 in. stainless steel sphere (superficial Rockwell) and evaluated in the HB, HV and HRC number scales. Thermal expansion along perpendicular and parallel directions to the Z was measured from room temperature to 1000uC in protective gas atmosphere with a commercial universal differential dilatometer. The sample size was 262640 mm and accuracy of
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Table 1 Physicochemical characteristics of transition metal carbide–graphite HMC materials Composition, vol.-%
Chemical analysis, wt-%
Density
Materials code
C/Me
MeC12x
C
Me*
C{
O
N
W
FezCo
g cm23
%%{
TKU-1 TKU-2 TKU-3 TKU-4 TKU-5 TKU-6 TsKU-1 TsKU-2 TsKU-3 TsKU-4 TsKU-5 GKU-1 GKU-2 GKU-3 GKU-4
1.33 1.68 2.13 2.59 3.28 4.43 1.31 1.76 2.33 2.99 3.89 1.31 1.69 2.19 2.86
87.4 77.2 66.9 59.0 50.1 40.0 90.2 79.0 68.5 59.4 50.1 89.6 79.8 70.0 59.9
12.6 22.8 33.1 41.0 49.9 60.0 9.8 21.0 31.5 40.6 49.9 10.4 20.2 30.0 40.1
73.5 68.9 64.1 59.7 54.0 46.7 83.7 79.2 74.9 70.7 65.0 89.9 87.7 85.0 82.0
24.5 29.0 34.3 38.8 44.5 51.9 14.4 18.4 23.0 27.8 33.3 7.9 10 12.5 15.8
0.6 0.6 0.6 0.6 … … 0.7 1.0 0.7 0.6 … … … … …
0.4 0.4 0.3 0.3 … … 0.5 0.4 0.4 0.3 … … … … …
0.4 0.4 0.2 0.2 … … 0.3 0.3 0.3 0.2 … … … … …
0.10 0.05 0.06 0.04 … … 0.03 0.02 0.02 0.01 … … … … …
4.54 4.29 4.01 3.65 3.25 2.88 5.89 5.46 5.00 4.46 3.82 10.88 9.80 8.63 7.43
99.4 99.9 99.7 95.7 90.9 87.0 95.1 95.7 95.4 92.1 86.0 94.0 92.9 90.5 87.6
*For the GKU materials it is HfzZr contents in sum. {Total carbon content. {Relative density in per cents of theoretical density calculated from XRD measurements for the components.
measurements carried out at heating rate of 5 K min21 with the Cr–Ni alloy as a reference material, was y1 mm. For estimation of the thermal shock resistance parameters of the HMC materials, the experimental data from the previous papers by Shabalin with co-authors34–37 were applied.
Results and discussion
group 4 transition metal carbides amatrix lie in the interval from 761026 to 9?5 61026 K21,39 whereas extremely anisotropic in thermal expansion behaviour graphite phase possesses completely different CTEs in both main crystallographic directions: 1?561026 K21 along the (00l) planes and 4561026 K21 along the normal to this planes,40 with a coefficient of anisotropy f5aI/aH
