Optimization to develop multiple response hardness

Int. Journal of Refractory Metals and Hard Materials 52 (2015) 159–164

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Short communication

Optimization to develop multiple response hardness and compressive strength of zirconia reinforced alumina by using RSM and GRA S. Renold Elsen ⁎, T. Ramesh Dept. of Mechanical Engineering, NIT Trichy, India

a r t i c l e

i n f o

Article history: Received 1 May 2015 Received in revised form 8 June 2015 Accepted 15 June 2015 Available online 18 June 2015 Keywords: Zirconia reinforced alumina Box–Behnken design Micro Vickers hardness Compressive strength Regression Multi response optimization Response Surface Methodology and Grey Relational Analysis

a b s t r a c t In this work the effect of powder forming process parameters of zirconia reinforced alumina composites on micro hardness and compressive strength was studied. The weight percentages of zirconia added to alumina, compaction pressure and sintering temperature are the process parameters selected for this analysis. Using Box– Behnken technique in Response Surface Methodology (RSM), seventeen experimental runs are developed. The sintering temperature and weight percentage of zirconia added to alumina are found to influence the responses. The influencing parameters were identified by using analysis of variance and Grey Relational Analysis (GRA). The regression model for both micro hardness and compressive strength are developed. The increasing amount of zirconia added to alumina matrix is found to enhance the compressive strength of the composite and reduces the hardness of composite. Also, multi response optimization to obtain higher hardness and compressive strength are done using both RSM and GRA. © 2015 Elsevier Ltd. All rights reserved.

1. Introduction Zirconia reinforced alumina (ZTA) are ceramic composites that are widely used for knee and hip joint prostheses [1] and as insert materials in machining tools [2]. The wide spectrum of applications has created more interest in the detailed study of the composite. To develop ZTA composite with higher reliability and improved performance detailed investigation of the various physical and mechanical characteristics is required. The final composite characteristics depend on the material characteristics of the matrix, reinforcements and the fabrication techniques. Material characteristics such as hardness, yield stress, endurance limit creep and resistance to crack growth can be predicted from indentation studies [3]. ZTA composites prepared by 5, 10 and 20 wt.% of zirconia reinforced with alumina by powder processing showed decreasing hardness. But, powder synthesized by colloidal processing showed decreasing hardness up to 10 wt.% and increased above 10 wt.% [4]. The powder injection molding process was used to prepare ZTA composites with different binders and sintering temperature of 1400 °C and 1600 °C soaking time of 2 hours. A maximum hardened value of 1582.4 HV was reported at the sintering temperature of 1600 °C [5]. The ZTA composites, green bodies were sintered from 1475 °C to 1575 °C with a temperature increment of 25 °C after the slip casting process. The hardness value decreased from 1900 HV10 to 1785 HV10 for sintering temperature of 1425 °C and 1575 °C ⁎ Corresponding author. E-mail address: [email protected] (S.R. Elsen).

http://dx.doi.org/10.1016/j.ijrmhm.2015.06.007 0263-4368/© 2015 Elsevier Ltd. All rights reserved.

respectively [6]. The hardness of pure alumina prepared by slip cast exhibited highest hardness by sintering at 1400 °C and 0.3 wt.% Al2O3doped and pure 3 mol% yttria-partially stabilized zirconia measured maximum hardness at 1600 °C sintering temperature [7]. The compressive strength under applied compressive load is related to the initiation of micro cracking at pores and other microscopic defects in the ceramic composites [8]. Compressed alumina particle of size 40 nm exhibited severe plastic deformation and particle size of 120 nm yielded by brittle failure when observed under transmission electron microscope (TEM) by [9]. Flacher et al. reported the improved compressive strength of alumina reinforced zirconia by the addition of zirconia [10]. Gang Liu et al. used ice template method to produce 35% porous ZTA with a compressive strength of 81 MPa by [11]. ZTA ceramics of porous nature with the maximum compressive strength of 26.87 MPa were obtained from gel-casting method combined with infiltration process [12]. The powder forming technique is a simple and effective method to produce near net shape products and is chosen in the study. By careful tailoring of powder forming process parameters such as weight percentage of zirconia added to alumina, compaction pressure and sintering temperature the required properties can be achieved. 2. Materials and method 2.1. Process parameter selection and specimen preparation The Al2O3 powders are blended in ball mill with yttria stabilized zirconia ZrO2 powders with different weight ratios of (5, 10 and 15%)

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respectively without the addition of a binder. Then it is compacted into green bodies in a 10 mm and 30 mm diameter circular cavity dies separately for various pressure of 120, 140 and 160 MPa respectively. The crystallite size of the starting powder was estimated from the XRD pattern given in Fig. 1 to be 40 and 56 nm for alumina and zirconia respectively. The cylindrical green compact was prepared with 10 mm diameter 30 mm height and 30 mm diameter, 12 mm height respectively. The shrinkage initiates from 1100 °C to 1150 °C for alpha-alumina and maximum density that is achieved at 1600 °C is reported by previous literatures. So the initial temperature of 1200 °C and 1600 °C is selected as the final temperature level for ZTA composite. The green bodies were sintered at temperatures of 1200 °C, 1400 °C and 1600 °C respectively based on the 17 runs generated by the Box–Behnken method. The rise in temperature per minute was kept as 5 °C with a soaking period of 4 h in a box furnace. Density, porosity and water absorption of the ZTA composite fabricated by the same process parameter is published elsewhere [13]. 2.2. Compressive strength and micro hardness studies The micro hardness study is carried out on a sample of 30 mm diameter and 10 mm height. The specimen surface is polished using polishing machine with a diamond paste. The optical microscope was used to observe the finished surface to avoid scratches and other micro defects which may affect the results. Vickers micro hardness testing was carried out using WOLPERT WILSON Instrument — Model 402 MVD facility as per ASTM C1327-08. Each specimen was tested at five different locations with a maximum load of 1 kgf force applied for 10 s dwell time. The average hardness values obtained from the five locations were chosen as the response. The specimens of varying diameter from 9.8 to 7.2 mm and standard height of 12 mm were used for uniaxial compression test ASTM C1424-10. The top and bottom surface of the specimen is made parallel and polished on which the compressive load is applied by universal testing machine TINIUS OLSEN 50 kN. The compression test was carried out with a strain rate of 1 × 10−4 s− 1 and digitalized outputs of each test were obtained which gives the load to deformation plot. The specimens were prepared according to the 17 experimental runs developed using Box–Behnken Technique based on the control parameters and levels. The variables and levels selected and corresponding mechanical characterization (responses) that were analyzed are provided in Table 1. 3. Conformation of compressive strength and micro Vickers hardness model by ANOVA

Table 1 Process design layout using Box Behnken design and test results. Run Variables

Responses

Composition of Pressure Temperature Compressive Vickers hardness strength ZrO2 in Al2O3

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

A

B

C

CS

MVH

Wt.%

MPa

°C

MPa

HV

10 15 5 10 15 5 10 10 15 5 10 10 10 5 15 10 10

100 130 130 160 100 160 130 130 160 100 130 130 130 130 130 100 160

1200 1200 1200 1200 1400 1400 1400 1400 1400 1400 1400 1400 1400 1600 1600 1600 1600

8.094853 12.80896 7.338466 9.073013 119.3591 71.9682 88.70822 96.85784 143.6895 88.27868 112.8129 113.4199 123.9988 237.514 292.054 283.753 264.161

122.93 ± 25. 66 ± 13.82 207.075 ± 23.62 129.8 ± 26.71 972.4 ± 15.1 1170 ± 62.93 1085 ± 2.83 1056 ± 40.79 1054.22 ± 44 1127 ± 21.69 1112 ± 30.1 1019 ± 23.8 1024.325 ± 50.21 1934 ± 17.7 1524 ± 1.54 1676.4 ± 11.6 1627.83 ± 4

compressive strength of the composite. The compaction pressure (B) has no influence on the compressive strength of the composite material. The compressive strength model for compressive strength is considered significant as the Model F-value is 138.06, also the probability of the model created by noise is only 0.01 percentages. From the analysis the weight percentage of zirconia in alumina (A), sintering temperature (C) and sintering temperature2 (C2) was identified as significant model terms as the Prob N F values is less than 0.05. Also the Prob N F value for sintering temperature is very minimal (b0.0001), so it is considered to be the most influential parameter of the three. The Lack of Fit F-value of 83.52 in the model explains the insignificance of relative to the pure error. Adequate precision of 46.027 suggests an adequate signal so that the model can be employed to navigate the design space. Similarly the predicted R-squared value of 0.9116 for the investigation is in harmony with adjusted R-squared value of 0.98471. The signal to noise ratio calculated by adequate precision is 37.181, which indicates an adequate signal as it measures more than four. The empirical relation in terms of coded factors and actual factors for compressive is given in Eqs. (1) and (2) CS ¼ 106:57 þ 20:35A þ 130:02C þ 32:78C2 þ 12:27AC:

The ANOVA that is used for the conformation study of compressive strength is given in Table 2. From the analyzed model, composition (A) and sintering temperature (C) are observed to influence the

Fig. 1. XRD pattern of starting powders alumina and zirconia.

ð1Þ

Table 2 ANOVA for response surface quadratic model compressive strength. Source

Sum of square

DF

Mean square

F-value

p-Value Prob b F

Model A—Composition B—Pressure C—Temperature AB AC BC A2 B2 C2 Residual Lack of Fit Pure error Cor total

1.49E5 4503.58 1.61 1.41E5 1.39 601.95 0.081 89.37 75.5 3441.53 840.6 827.39 13.21 1.50E5

9 1 1 1 1 1 1 1 1 1 7 3 4 16

16,579.58 4503.58 1.61 1.41E5 1.39 601.95 0.081 89.37 75.5 3441.53 120.09 275.8 3.3 –

138.06 37.5 0.013 1169.95 0.012 5.01 6.77E-4 0.74 0.63 28.66 – 83.52 – –

b0.0001 0.0005 0.9109 b0.0001 0.9173 0.0602 0.98 0.4169 0.4539 0.0011 – 0.0005 – –

Standard deviation:—10.9583, R-squared:—0.9943, mean:—126.7249, adjusted Rsquared:—0.9871, C.V. %:—8.6473, predicted R-squared:—0.9116, PRESS:—13,258.9299, adequate precision:—37.181.

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Sum of square

DF

Mean square

F-value

p-Value Prob b F

Model A—Composition B—Pressure C—Temperature AB AC BC A2 B2 C2 Residual Lack of Fit Pure error Cor total

5,032,364.484 85,114.22258 13.36445 4,867,863.023 6.5025 22,339.03891 14.8225 5692.576581 32.05958059 52,784.42603 6641.238606 6496.364106 144.8745 5,039,005.722

9 1 1 1 1 1 1 1 1 1 7 3 4 16

559,151.6093 85,114.22258 13.36445 4,867,863.023 6.5025 22,339.03891 14.8225 5692.576581 32.05958059 52,784.42603 948.7483723 2165.454702 36.218625 –

589.357121 89.7121145 0.0140864 5130.82622 0.00685377 23.5457995 0.01562322 6.00009101 0.03379145 55.6358541 – 59.7884294 – –

b0.0001 b0.0001 0.9088 b0.0001 0.9363 0.0018 0.9040 0.0441 0.8593 b0.0001 – 0.00088 – –

Standard deviation:—30.8017, R-squared:—0.9986, mean:—978.78, adjusted R-squared:—0.9969, C.V. %:—3.1469, predicted R-squared:—0.9793, PRES:—104,168.1921, adequate precision:—75.269.

Compressive Strength (MPa)

310 248 186 124 62 0

1600.00 15.00

1520.00

13.00

1440.00

C: Temperature (C)

11.00

1360.00

9.00

1280.00

7.00 1200.00

5.00

A: composition (wt%)

Fig. 2. Compressive strength response plot based on the composition of zirconia and sintering temperature.

CS ¼ 933:86‐13:1Composition‐1:76Temperature þ 8:19  10‐4 Temperature2 þ 0:012Composition  Temperature:

4. Influence of process parameter on compression strength and micro hardness

ð2Þ The conformation study of Vickers hardness by ANOVA is given in Table 3. The F-value of the micro Vickers hardness model was predicated by ANOVA to be 589.357121 which exhibit the significance of the model. The possibility of F-value this large that can occur due to noise is 0.01 percentages. From the analysis Prob N F value smaller than 0.05 is found for the weight percentage of zirconia in alumina (A), sintering temperature (C) combined effect of (A and C) and sintering temperature2 (C2) which indicates the significant model terms. The compaction pressure (B) and other interaction terms have no influence on the micro Vickers hardness of the composite material. The predicted R-squared value of 0.9793 is in agreement with an adjusted R-squared value of 0.9969. The adequate precision value 75.269 measures the signal to noise ratio, is greater than 4 is desirable and the model can be used to navigate the design space. The Eq. (3) in terms of coded factors and Eq. (4) with actual factors can be used to predict the response to the provided levels of each factor. MVH ¼ 1068:88‐102:68A þ 779:55C‐67:23  A  C‐157:88C2 :

ð3Þ

MVH ¼ ‐12859:86 þ 73:58composition þ 15:62Temperature‐0:067231 composition  Temperature −3:94  10‐3 Temperature2 :

ð4Þ

The coupled effect of the composition of zirconia and sintering temperature on the compressive strength of the zirconia reinforced alumina ceramic are shown in Fig. 2. The compressive strength is

Fig. 3. HR SEM image of ZTA sintered at 1200 °C.

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addition of ZrO2 in the matrix causes the zirconia ions (Zr4+) to hinder the grain boundary sliding of alumina [10]. Also, due to pinning effect ZrO2 checks the grain growth of alumina during sintering which is also a possible strengthening effect of the composite. Compressive strength is enhanced from 7.33 MPa to 292.05 MPa for increasing sintering temperature from 1200 °C to 1600 °C respectively. At 1200 °C sintering temperature, more voids were observed (Fig. 3) and also the grain boundary diffusion between the particles is minimal, this causes immediate failure and minimal compressive strength. The 1400 °C sintered sample has shown comparatively improved grain boundary diffusion between the particles with voids (Fig. 4). Also higher compressive strength was observed compared to 1200 °C sintered samples. At 1600 °C, proper grain boundary diffusion with few voids is observed as shown in Fig. 5. The collective effects of both weight percentage of zirconia added to alumina and sintering temperature on micro Vickers hardness of the zirconia that reinforced alumina ceramic are given in Fig. 6. The sintering temperature is found to be the most influential factor, and the hardness value increases with the increase in temperature. This is due to the fact that 98% theoretical density with less porosity is achieved at 1600 °C. From the response plot, it is observed that the hardness is increased from 96 HV at 1200 °C to 1860 HV at 1600 °C for 5 wt.% of zirconia added to alumina. The hardness value is found to be reduced by the addition of zirconia also predicted by the response surface plot and similar behavior was reported by Arab et al. [14]. However, the hardness is decreased from 1860 HV at 5 wt.% of zirconia to 1524 HV for 15 wt.% of zirconia at 1600 °C by 18%. The plot shows a gradual decrease in the response as the weight percentage of zirconia added to alumina factor is increased. The composite prepared by the powder injection molding process with 95% theoretical density exhibited 1582.4 HV [5]. A maximum hardness of 1934 HV has been achieved for 96.5% theoretical density of the final composite material.

Fig. 4. HR SEM image of ZTA sintered at 1400 °C.

5. Optimization of process parameters using RSM and GRA 5.1. Response Surface Methodology (RSM)

Fig. 5. HR SEM image of ZTA sintered at 1600 °C.

increased from 226 MPa to 302 MPa at 1600 °C sintering temperature with the addition of zirconia to alumina from 5 wt.% to 15 wt.% respectively by 33%. The increase in the composition of zirconia added to alumina increases the compressive strength of the composite. The

Multi response optimization is done using Response Surface Methodology (RSM), a widely accepted design of experiment method and using Grey Relational Analysis. RSM is a statistical method used to represent a phenomenon, assess the performance of any individual or collective effect of factors on responses and also to optimize the factors [15]. The constraints and the limits considered are given in Table 4. The weight percentage of zirconia has a limit of 5 to 15 percentages, a

Micro Vickers Hardness (Hv)

1860.5

1419.38

978.25

537.125

96

1600.00

15.00 1520.00

13.00 1440.00

11.00 1360.00

C: Temperature (C)

9.00 1280.00

7.00 1200.00

A: composition (wt%)

5.00

Fig. 6. Interaction effects of composition and temperature on micro hardness.

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S.R. Elsen, T. Ramesh / Int. Journal of Refractory Metals and Hard Materials 52 (2015) 159–164 Table 4 Constraints and limits for compressive strength and Vickers micro hardness. Constraints

Table 7 Response table for Grey relational grade.

Limits

Name

Goal

Lower limit

Upper limit

A: Composition B: Compaction pressure C: Temperature Compressive strength Vickers hardness

Is in range Is in range Is in range Maximum Maximum

5 (wt.% of zirconia) 100 MPa 1200 °C 292.054 (MPa) 1934 (HV)

15 (wt.% of zirconia) 160 MPa 1600 °C

Process parameter

A

B

C

Level 1 Level 2 Level 3 Range (maximum–minimum value) Order

0.537241 0.53939* 0.523843 0.015547 2

0.53903* 0.53733 0.526566 0.010764 3

0.338162 0.477832 0.841994* 0.503832 1

“ ”

* indicates the optimal levels of the process parameter.

Table 8 Comparison of confirmation experiments with the results. Table 5 Optimized value for responses and highest value of desirability. Process parameter

Response C

Compressive strength

Micro Vickers hardness

MPa

°C

MPa

HV

100.001

1600

292.054

1623.828

A

B

Wt.% 11.815 “ͼ”

ͼ

Exp. no

Desirability

0.912

indicates that the factor compaction pressure has no effect on the response.

compaction pressure of 100 MPa to 160 MPa and sintering temperature 1200 °C to 1600 °C are taken and goals were set for maximum compressive strength and Vickers hardness. The predicted process parameter of 11.815 wt.% of zirconia added to alumina, 100.001 MPa pressure and 1600 °C sintering temperature for a maximum response was 292.054 MPa, compressive strength and 1623.828 Vickers Hardness with desirability of 0.898 from the optimized results (Table 5). The factor compaction pressure has no effect on the response which is due to insufficient magnitude of pressure. The same effect is predicted by ANOVA that the pressure has no influence.

1 2 3

Compressive strength (MPa)

Vickers hardness (HV)

Obtained

Predicated

Error

Obtained

Predicated

Error

273.0705 276.0202 264.0168

292.054 292.054 292.054

6.5% 5.49% 9.6%

1575.113 1581.608 1560.499

1623.83 1623.83 1623.83

3% 2.6% 3.9%

by using the higher the better condition is given by Eqs. (5) and (6). NðCSi Þ ¼

CSi ‐ minðCSi Þ : maxðCSi Þ‐ minðCSi Þ

NðMVHi Þ ¼

ð5Þ

MVHi ‐ minðMVHi Þ : maxðMVHi Þ‐ minðMVHi Þ

ð6Þ

The Δoi(CS) and Δoi(MVH) are the deviation sequence of the reference sequence N(CS) and N(MVH) respectively given by Eqs. (7) and (8). Δoi ðCSÞ ¼ 1‐NðCSi Þ:

ð7Þ

Δoi ðMVHÞ ¼ 1‐NðMVHi Þ:

ð8Þ

5.2. Grey Relational Analysis The Grey analysis is used to characterize the grade of correlation inbetween two sequences to measure the space between two factors. Grey Relation Analysis is a useful method to analyze the correlation between sequences with minimal data where experiments are time consuming and expensive [16]. It can also analyze multiple factors, which are the disadvantages of statistical method. In the Grey relational generating process the responses are normalized in the range of 0 and 1

Grey relational coefficient is determined to assess the association of the ideal with actual normalized experimental responses. The Grey relation coefficient ξi(CS) and ξi(MVH) for the ith experiment can be expressed as and are given by Eqs. (9) and (10). ξi ðCSÞ ¼

Δoi ðCSmin Þ þ ζ Δoi ðCSmax Þ : Δoi ðCSÞ þ ζ Δoi ðCSmax Þ

ξi ðMVHÞ ¼

ð9Þ

Δoi ðMVHmin Þ þ ζ Δoi ðMVHmax Þ : Δoi ðMVHÞ þ ζ Δoi ðMVHmax Þ

ð10Þ

Table 6 Calculated normalized value Grey relational co-efficient, grade values and rank of process design layout. Run

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

Composition of ZrO2 in Al2O3

Pressure

Temperature

Reference sequence

Deviation sequence

GRC after weighted

(Wt.% of ZrO2)

(MPa)

(°C)

N(CS)

N(MVH)

Δoi(CS)

Δoi(MVH)

CS

MVH

2 3 1 2 3 1 2 2 3 1 2 2 2 1 3 2 2

1 2 2 3 1 3 2 2 3 1 2 2 2 2 2 1 3

1 1 1 1 2 2 2 2 2 2 2 2 2 3 3 3 3

0.00266 0.01921 0 0.00609 0.39345 0.227 0.28579 0.31442 0.4789 0.28428 0.37046 0.37259 0.40974 0.80844 1 0.97084 0.90203

0.05204 0 0.11086 0.06109 0.60407 0.72851 0.67647 0.66063 0.6086 0.71041 0.66968 0.67647 0.6629 1 0.81448 0.9095 0.89367

0.99734 0.98079 1 0.99391 0.60655 0.773 0.71421 0.68558 0.5211 0.71572 0.62954 0.62741 0.59026 0.19156 0 0.02916 0.09797

0.947964 1 0.88914 0.938914 0.395928 0.271493 0.323529 0.339367 0.391403 0.289593 0.330317 0.323529 0.337104 0 0.18552 0.090498 0.106335

0.3339 0.3377 0.3333 0.3347 0.4519 0.3928 0.4118 0.4217 0.4897 0.4113 0.4427 0.4435 0.4586 0.7230 1.0000 0.9449 0.8362

0.3402 0.3333 0.3510 0.3411 0.4927 0.5501 0.5238 0.5155 0.5149 0.5365 0.5319 0.5051 0.5066 1.0000 0.6949 0.7838 0.7531

GRG

Rank

0.3371 0.3355 0.3422 0.3379 0.4723 0.4714 0.4678 0.4686 0.5023 0.4739 0.4873 0.4743 0.4826 0.8615 0.8475 0.8644 0.7946

16 17 14 15 10 11 13 12 5 9 6 8 7 2 3 1 4

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The ζ is distinguished or identification coefficient in this work is 0.5 because all of the responses were given equal weight. The average of the Grey relational coefficient is the Grey relational grade given by Eq. (11). γi ¼

n 1X fξ ðCSÞ þ ξi ðMVHÞg: n i¼1 i

ð11Þ

The largest Grey relation grade is chosen as an optimal process parameter setting for the experiment which corresponds to the 16 run. From the performed GRA Table 6 the process parameters' setting of experiment number 16 has the highest Grey relational grade. Based on the Grey relational grade values, the optimal parameter for maximum compressive strength and Vickers hardness was obtained from 1600 °C sintering temperature (level 3), 10 wt.% of zirconia in alumina (level 2), and 100 MPa compaction pressure (level 1) combination. The order of importance of the process parameter to the multi response characteristics in the process parameter, in sequence is given in Table 7 as: factor C (sintering temperature), which was the most dominant process parameter factor A (weight percentage of zirconia in alumina) and B (compaction pressure) i.e. 0.503833 N 0.015547 N 0.010764. ANOVA also predicted the similar effect of process parameter on the response. The optimal levels of the three process parameters are the weight percentage of zirconia added to alumina at level 2 (10 wt.%), compaction pressure at level 1 (100 MPa) and finally sintering temperature at level 3 (1600 °C). Similar prediction were also observed from optimization done using RSM. 6. Conformation studies Three conformation experimental runs were conducted for the optimal predicted process parameter predicted by using RSM. Three experimental procedures were repeated and the test was conducted as mentioned previously. The obtained value is found to have a good agreement with the predicted value and the estimated error is between 5.49 and 9.6% for compressive strength (Table 8). The Vickers hardness error values are found between 4 and 2.6% which are minimal. 7. Conclusion The mechanical characteristic studies such as compressive strength and micro Vickers hardness of ZTA composite prepared by powder forming process were analyzed in this work. • The factors which influence the response and the optimization of the process parameters were identified and compared by using ANOVA, Grey Relational Analysis methods. • The results showed that the temperature is the most influential factor in the selected design space, which improves the compressive strength and Vickers hardness of the composite followed by composition of zirconia added to alumina. • With raise in sintering temperature from 1200 °C to 1600 °C, there is a good improvement in the compressive strength and hardness due to minimal porosity and good grain boundary diffusion at 1600 °C. • The increase in weight percentage of zirconia in alumina matrix from 5 to 15 has increased the compressive strength by 33% and reduced the micro hardness of the composite by 18%.

• The factor compaction pressure showed no influence on the responses because the magnitude of the applied pressure is insufficient to cause any microstructural change in the composite. • The optimum process parameters of 11.815 wt % (A), 100 MPa (B) and 1600 °C (C) are obtained from RSM. • Also, optimization is done to obtain higher compressive strength and hardness using RSM and GRA and conformation experimental runs are conducted and the results with validated least error.

Acknowledgement The authors wish to thank Dr. P. Senthil, Assistant Professor, Production Engineering, NIT — Trichy for extending the micro Vickers hardness facility to carry out the tests which were procured by funding project reference No (SR/FT/ET-033/2009).

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