E噩≈S Journal of the European Ceramic Society 23(2003)309-313 www.elsevier.com/locate/jeurceramsoc Thermal shock properties of alumina reinforced with TI(C, N whiskers Pernilla pettersson Mats johnsson* Department of Inorganic Chemistry, Stockholm University, S-106 91 Stockholm, Sweden Received I March 2002; received in revised form 26 May 2002: accepted I June 2002 Abstract a Alumina composites reinforced with Ti(C, N)whiskers were produced to evaluate the thermal shock properties. The indentation acture toughness (49 N load)increased from 2.6 MPa m/ for pure alumina to 5.0 MPa m/ for the sample with 30 vol% Ti(C, N)whiskers. The hardness also increased, from 17.6 to 24.2 GPa. a clear R-curve behaviour was observed. An indentation- quench test was used to measure the thermal shock resistance. The best thermal shock resistance was observed at 30 vol. Ti(C, N C 2002 Elsevier Science Ltd. All rights reserved Keywords: Al2O3; Composites; Indentation; Thermal shock resistance; Ti(C, N); Whiskers 1. Introduction on indenting small initial cracks on a polished plate. The indented plate is heated in a vertical tube furnace Alumina is a brittle material with a poor thermal and subsequently quenched in a water bath. The crack shock resistance. However, the mechanical properties length is measured before and after quenching. The can be improved substantially by reinforcing it with a indentation-quench method gives data useful for com- phase having good thermal stability, high strength and paring the thermal shock resistance of different ceramic high elastic modulus. Sic whiskers is the preferred materials. The influence of different experimental para- reinforcing phase, especially for cutting tool applica meters on the resolution of this measurement technique tions. During the last few years a number of new whis- has recently been evaluated ker materials in the form of transition metal carbide and The aim of the present work was to evaluate the carbonitride phases have been developed, such as Tic thermal shock resistance of alumina composites rein- nd Ti(C, N ) ,2 Such whiskers have high hardness and forced with Ti(C, N) whiskers. Another aim was to test strength, and they are chemically inert to iron up to the applicability of the indentation-quench test to this high temperatures, making them interesting candidates type of materials for reinforcement of ceramic cutting tools. Many thermal-shock testing techniques use test bars of specified dimensions and geometries. After a heating 2. Experimental and quenching procedure the bars are subjected to mechanical testing by e.g. three- or four-point bending- 2.1. Starting materials and sample preparation strength tests. These techniques involve some draw backs: a new test bar is needed for each temperature The Ti(C, N) whiskers were synthesised carbother and, to improve statistics, more than one bar should be mally via a vapour-liquid-solid (VLS) growth mechan ested at each temperature. The technique used in the Ism ace cording to Ahlen et al. l, 2 The whisker product current work is an indentation-quench method based was analysed chemically for Ti, N, C and O, and the bulk composition was found to be TiCo.21 No6 g Oo. 16. The 4 Corresponding author whisker morphology was investigated both in an optical E-mail address: mats@ inorg. suse(M. Johnsson). microscope and in a scanning electron microscope 0955-2219/02/S. see front matter C 2002 Elsevier Science Ltd. All rights reserved. PII:S0955-2219(02)00177-2
Thermal shock properties of alumina reinforced with Ti(C,N) whiskers Pernilla Pettersson, Mats Johnsson* Department of Inorganic Chemistry, Stockholm University, S-106 91 Stockholm, Sweden Received 1 March 2002; received in revised form 26 May 2002; accepted 1 June 2002 Abstract Alumina composites reinforced with Ti(C,N) whiskers were produced to evaluate the thermal shock properties. The indentation fracture toughness (49 N load) increased from 2.6 MPa m1=2 for pure alumina to 5.0 MPa m1=2 for the sample with 30 vol.% Ti(C,N) whiskers. The hardness also increased, from 17.6 to 24.2 GPa. A clear R-curve behaviour was observed. An indentation– quench test was used to measure the thermal shock resistance. The best thermal shock resistance was observed at 30 vol.% Ti(C,N) whiskers. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Al2O3; Composites; Indentation; Thermal shock resistance; Ti(C,N); Whiskers 1. Introduction Alumina is a brittle material with a poor thermal shock resistance. However, the mechanical properties can be improved substantially by reinforcing it with a phase having good thermal stability, high strength and high elastic modulus. SiC whiskers is the preferred reinforcing phase, especially for cutting tool applications. During the last few years a number of new whisker materials in the form of transition metal carbide and carbonitride phases have been developed, such as TiC and Ti(C,N).1,2 Such whiskers have high hardness and strength, and they are chemically inert to iron up to high temperatures, making them interesting candidates for reinforcement of ceramic cutting tools. Many thermal-shock testing techniques use test bars of specified dimensions and geometries. After a heating and quenching procedure the bars are subjected to mechanical testing by e.g. three- or four-point bendingstrength tests. These techniques involve some drawbacks: a new test bar is needed for each temperature and, to improve statistics, more than one bar should be tested at each temperature. The technique used in the current work is an indentation–quench method3 based on indenting small initial cracks on a polished plate. The indented plate is heated in a vertical tube furnace and subsequently quenched in a water bath. The crack length is measured before and after quenching. The indentation–quench method gives data useful for comparing the thermal shock resistance of different ceramic materials. The influence of different experimental parameters on the resolution of this measurement technique has recently been evaluated.4 The aim of the present work was to evaluate the thermal shock resistance of alumina composites reinforced with Ti(C,N) whiskers. Another aim was to test the applicability of the indentation–quench test to this type of materials. 2. Experimental 2.1. Starting materials and sample preparation The Ti(C,N) whiskers were synthesised carbothermally via a vapour–liquid–solid (VLS) growth mechanism according to Ahle´n et al.1,2 The whisker product was analysed chemically for Ti, N, C and O, and the bulk composition was found to be TiC0.21N0.63O0.16. The whisker morphology was investigated both in an optical microscope and in a scanning electron microscope 0955-2219/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S0955-2219(02)00177-2 Journal of the European Ceramic Society 23 (2003) 309–313 www.elsevier.com/locate/jeurceramsoc * Corresponding author. E-mail address: matsj@inorg.su.se (M. Johnsson)
P. Pettersson, M. Johnsson /Journal of the European Ceramic Society 23(2003)309-313 (SEM, JEOL 820). The whisker yield is estimated to be Table I 80 vol. and the remaining 20 voL. is particles of the Density, hardness, and fracture toughness same compound; the whiskers have a length of 30-40 Reinforcing Volume Density H um and a diameter in the range 1-3 um. The matrix phase GPa) (MPa m/2) material was-Al2O3(AKP-30, Sumitomo Chemical Composites containing 5-40 vol. Ti(C, N)whiskers Ti(C Nw were prepared. In order to suppress alumina grain Ti(C, N)w growth, 0.25 wt. Mgo was added(by AlO3 dry Ti(C, Nw 15 weight). A slurry of Al2O3, Mg(NO3)2 6H,O, and polyacrylic acid dispersant(Dispex A40, Allied Col- Ti(C, N)w Ti(C, N)w 09999999 lords, USA)was mixed by ball milling with Al,O cylpebs as milling media in deionised water for Ti(C N)w 998 22.4 approximately 18 h. The reinforcing phase was then added to the slurry and the milling was continued for another 5 h. The slurry was then instantly frozen in liquid nitrogen and subsequently freeze-dried Table 2 Before sintering the composite mixtures were heat Thermal shock parameters: sample thickness, indentation load, and initial crack lengt reated at 600C( h)in Ar-5% H atmosphere to remove residues of Dispex A40 and to decompose Reinforcing Amount Sample Load Initial crack Mg(NO3)2- 6H,0 to Mgo before hot pressing phase (vol %) thickness (M (um) Green bodies with a diameter of o=12 mm were prepared and sintered in a hot-press furnace(Thermal TiC Nw 5 4.07 Ti(C, Nw Ti(C, Nw 2. 2. Characterisation Ti(C, Nw Ti(C, Nw The density of all sintered samples was measured by TI(C, Nw 4.17 use of Archimedes'principle. Before physical char- Ti(C, Nw 4.04 acterisation the specimens were carefully polished by Ti(C N)w standard diamond polishing techniques down to a dia mo nd particle size of The microstructure of the composites, in sections both (612 GPa). Youngs modulus for the different compo- parallel and perpendicular to the pressure directions, sites was estimated by assuming a linear relation was investigated with a scanning electron microscope between the values for Al2O3(380 GPa) and the rein- (SEM, JEOL 820). To obtain the best contrast, the forcing phase. The R-curve behaviour for two of the micrographs were recorded in back-scattered electron whisker composites was determined with loads in the mode(bse) range 35-98 N The hardness (H) and fracture toughness (Klc)at room temperature were evaluated by the Vickers inden 23. Thermal shock measurements tation technique at a load of 49 N for all compositions see table l five indents were made in a row at the mid Vickers indents were introduced into disc-shaped dle(to minimise near-surface effects)of each sample. The samples(0=12 mm, h=4 mm) with parallel surfaces, fracture toughness was calculated by the indentation one of them polished. Each indent generates four method according to Anstis et al., 6 see Eq. (I) cracks. and four indents were introduced on each sam ple, giving a total of 16 cracks per sample. The crack c3/2 () length is defined as the distance from the centre of the indent to the crack tip. To facilitate comparison between different samples, the initial crack length() A is a constant for Vickers-produced radial cracks(a was held at approximately 100 um, meaning that the value of 0.016 has been used), E is Youngs modulus, H indentation load was varied, see Table 2. The cracks is the hardness, P is the load and c is the crack length. were measured in an optical microscope (Olympus For the calculations a value for Youngs modulus of the PMG3). Each one was monitored individually, and the whiskers was estimated for a whisker composition of total crack length after thermal shock (T) was measured iCo.25 No75 by assuming a linear relation between the and the percentage crack growth(Ac)was calculated Youngs modulus values for TiC (451 GPa)and TiN using Eq (2). If one or two cracks deviated from the
(SEM, JEOL 820). The whisker yield is estimated to be 80 vol.% and the remaining 20 vol.% is particles of the same compound; the whiskers have a length of 30–40 mm and a diameter in the range 1–3 mm. The matrix material was -Al2O3 (AKP-30, Sumitomo Chemical, Inc.). Composites containing 5–40 vol.% Ti(C,N) whiskers were prepared. In order to suppress alumina grain growth, 0.25 wt.% MgO was added (by Al2O3 dry weight).5 A slurry of Al2O3, Mg(NO3)2 .6H2O, and polyacrylic acid dispersant (Dispex A40, Allied Colloids, USA) was mixed by ball milling with Al2O3 cylpebs as milling media in deionised water for approximately 18 h. The reinforcing phase was then added to the slurry and the milling was continued for another 5 h. The slurry was then instantly frozen in liquid nitrogen and subsequently freeze-dried. Before sintering the composite mixtures were heat treated at 600 C (1 h) in Ar–5% H2 atmosphere to remove residues of Dispex A40 and to decompose Mg(NO3)2 .6H2O to MgO before hot pressing. Green bodies with a diameter of Ø=12mm were prepared and sintered in a hot-press furnace (Thermal Technology Inc.) at 1700 C, 28 MPa, for 90 min. 2.2. Characterisation techniques The density of all sintered samples was measured by use of Archimedes’ principle. Before physical characterisation the specimens were carefully polished by standard diamond polishing techniques down to a diamond particle size of 1 mm. The microstructure of the composites, in sections both parallel and perpendicular to the pressure directions, was investigated with a scanning electron microscope (SEM, JEOL 820). To obtain the best contrast, the micrographs were recorded in back-scattered electron mode (BSE). The hardness (H) and fracture toughness (KIc) at room temperature were evaluated by the Vickers indentation technique at a load of 49 N for all compositions, see Table 1. Five indents were made in a row at the middle (to minimise near-surface effects) of each sample. The fracture toughness was calculated by the indentation method according to Anstis et al.,6 see Eq. (1). K1C ¼ A E H 1=2 P c3=2 ð1Þ A is a constant for Vickers-produced radial cracks (a value of 0.016 has been used7 ), E is Young’s modulus, H is the hardness, P is the load and c is the crack length. For the calculations a value for Young’s modulus of the whiskers was estimated for a whisker composition of TiC0.25N0.75 by assuming a linear relation between the Young’s modulus values for TiC (451 GPa) and TiN (612GPa). Young’s modulus for the different composites was estimated by assuming a linear relation between the values for Al2O3 (380 GPa) and the reinforcing phase. The R-curve behaviour for two of the whisker composites was determined with loads in the range 35–98 N. 2.3. Thermal shock measurements Vickers indents were introduced into disc-shaped samples (Ø=12mm, h=4 mm) with parallel surfaces, one of them polished. Each indent generates four cracks, and four indents were introduced on each sample, giving a total of 16 cracks per sample. The crack length is defined as the distance from the centre of the indent to the crack tip. To facilitate comparison between different samples, the initial crack length (l ) was held at approximately 100 mm, meaning that the indentation load was varied, see Table 2. The cracks were measured in an optical microscope (Olympus PMG3). Each one was monitored individually, and the total crack length after thermal shock (lT) was measured and the percentage crack growth (c) was calculated, using Eq. (2). If one or two cracks deviated from the Table 1 Density, hardness, and fracture toughness Reinforcing phase Volume fraction (%) Density (%) H (GPa) KIc (MPa m1=2 ) – 0 100.0 17.6 2.6 Ti(C,N)W 5 99.5 17.9 2.9 Ti(C,N)W 10 99.6 18.6 3.4 Ti(C,N)W 15 99.7 19.24.2 Ti(C,N)W 20 99.9 20.3 4.5 Ti(C,N)W 25 99.6 22.6 4.9 Ti(C,N)W 30 99.7 24.2 5.0 Ti(C,N)W 35 99.6 23.6 5.0 Ti(C,N)W 40 99.8 22.4 4.7 Table 2 Thermal shock parameters: sample thickness, indentation load, and initial crack length Reinforcing phase Amount (vol.%) Sample thickness (mm) Load (N) Initial crack length (mm) – 0 3.90 40 110 Ti(C,N)W 5 4.07 58 128 Ti(C,N)W 10 3.87 58 121 Ti(C,N)W 15 4.15 58 92 Ti(C,N)W 20 4.05 58 101 Ti(C,N)W 25 4.15 58 104 Ti(C,N)W 30 4.17 58 95 Ti(C,N)W 35 4.11 58 97 Ti(C,N)W 40 4.04 58 107 310 P. Pettersson, M. Johnsson / Journal of the European Ceramic Society 23 (2003) 309–313
P. Pettersson, M. Johnsson /Journal of the European Ceramic Society 23(2003)309-313 growth of the others. the Students t-test at 95% con- 3. Results and discussion fidence was used as criteria to check if they should be included in the calculation of the average crack 3.1. Microstructure and mechanical properties The whiskers were homogeneously distributed up to △c= (2) 30-35 vol. reinforcing phase. However, for the sample containing 40 vol %o whiskers some whisker agglomer ates were found, see Fig. la-c. This fact may be the Each sample was hoisted into a vertical tubular fur- reason for the best mechanical properties being found at nace and heated to a predetermined temperature (Te). an addition of 30 vol. reinforcing phase, see discus After 20 min at that temperature the sample was quen sion below. During sintering, whiskers arrange them ched in a water bath at Tw=90C, and the cracks were selves perpendicular to the pressure direction. All measured to evaluate the growth samples were found to be fully dense Both hardness (H) and fracture toughness (Kl) △T=TF-Tw (3) increase with increasing fraction of reinforcing phase up Water close to the boiling point offers quite mild toughness decrease again, see Table/ess and fracture quenching conditions. 4. 8 The heating and quenching The alumina sample without any reinforcing phase procedure was repeated with the same sample at step las a hardness value of 17.6 GPa and fracture tough- wise higher and higher T values. It has been shown that ness of 2.6 MPa m, which may be compared to the the indentation-quench technique allows the same sam- sample with 30 vol. Ti(C, N)whiskers having a hardness ple to be used for a whole quenching series when the of 24.2 GPa and a fracture toughness of 5.0 MPa m/ aim is to rank the thermal shock properties of different This measured fracture toughness, however, is lower materials. The samples were heated in air. At the high- than what is generally reported for Al2O3 reinforced with est temperature used in the experiments, 590C, a slight 30 voL. SiC whiskers. The fracture toughness is mea- degree of oxidation was observed on the surface of the sured to be 5-7.7 MPa m/ for this type of materials, reinforcing phase, which was not considered severe using Anstis' indentation method A useful thermal shock parameter is ATx,corre- sponding to the temperature difference inducing X% 3. 2. Thermal shock properties growth of the initial cracks. Thus, ATlo is the thermal shock temperature difference making the cracks grow by Examples of the thermal shock behaviour at different 10% of their initial length quenching temperature differences(AT)for the whisker 10 um Fig. I. SEM micrographs recorded in backscattered electron mode Composites with(a)10 vol %, (b)20 vol. and(c)40 vol %Ti(C N)whisker The reinforcing phase appears white and the matrix phase black or dark grey
growth of the others, the Student’s t-test at 95% con- fidence was used as criteria to check if they should be included in the calculation of the average crack growth. c ¼ lT l l 100 ð2Þ Each sample was hoisted into a vertical tubular furnace and heated to a predetermined temperature (TF). After 20 min at that temperature the sample was quenched in a water bath at TW=90 C, and the cracks were measured to evaluate the growth. T ¼ TF TW ð3Þ Water close to the boiling point offers quite mild quenching conditions.4,8 The heating and quenching procedure was repeated with the same sample at stepwise higher and higher T values. It has been shown that the indentation–quench technique allows the same sample to be used for a whole quenching series when the aim is to rank the thermal shock properties of different materials.4 The samples were heated in air. At the highest temperature used in the experiments, 590 C, a slight degree of oxidation was observed on the surface of the reinforcing phase, which was not considered severe. A useful thermal shock parameter is TX, corresponding to the temperature difference inducing X% growth of the initial cracks. Thus, T10 is the thermal shock temperature difference making the cracks grow by 10% of their initial length. 3. Results and discussion 3.1. Microstructure and mechanical properties The whiskers were homogeneously distributed up to 30–35 vol.% reinforcing phase. However, for the sample containing 40 vol.% whiskers some whisker agglomerates were found, see Fig. 1a–c. This fact may be the reason for the best mechanical properties being found at an addition of 30 vol.% reinforcing phase, see discussion below. During sintering, whiskers arrange themselves perpendicular to the pressure direction. All samples were found to be fully dense. Both hardness (H ) and fracture toughness (KIc) increase with increasing fraction of reinforcing phase up to 30 vol.%, and then both hardness and fracture toughness decrease again, see Table 1. The alumina sample without any reinforcing phase has a hardness value of 17.6 GPa and fracture toughness of 2.6 MPa m1=2 , which may be compared to the sample with 30 vol.% Ti(C,N) whiskers having a hardness of 24.2 GPa and a fracture toughness of 5.0 MPa m1=2 . This measured fracture toughness, however, is lower than what is generally reported for Al2O3 reinforced with 30 vol.% SiC whiskers. The fracture toughness is measured to be 5–7.7 MPa m1=2 9 for this type of materials, using Anstis’ indentation method. 3.2. Thermal shock properties Examples of the thermal shock behaviour at different quenching temperature differences (T ) for the whisker Fig. 1. SEM micrographs recorded in backscattered electron mode. Composites with (a) 10 vol.%, (b) 20 vol.% and (c) 40 vol.% Ti(C,N) whiskers. The reinforcing phase appears white and the matrix phase black or dark grey. P. Pettersson, M. Johnsson / Journal of the European Ceramic Society 23 (2003) 309–313 311
P. Pettersson, M. Johnsson /Journal of the European Ceramic Society 23(2003)309-313 composites are given in Fig. 2. The thermal shock resis- thermal shock resistance, see Fig 3. The volume fraction tance increases with increasing amount of whiskers up to a of whiskers giving the best thermal shock resistance (30 fraction of 30%, where the optimum thermal vol %)is also found in commercially available ceramic- properties are observed. A further increase in the cutting tools based on Sic-whisker reinforced alumina. volume fraction of whiskers leads to a decrease in the The measurements show that the thermal shock resis- tance improves with increasing fracture toughness. Such a relation has also been observed for other brittle materials. 0 A clear R-curve behaviour is observed for 一·20vol the Ti(C, N)whisker-reinforced composites, see Fig. 4. -v 30 vol. Comparing the error bars in Fig. 2, it is clear that the 口40vol.% catter is largest at about 10 vol. whiskers. This can be explained by that such a material have properties intermediate between pure alumina and a whisker-rein- forced composite. With only 10 vol. whiskers present some of the cracks will grow solely in the alumina matrix, without meeting any whisker, while some cracks will meet whiskers that absorb crack energy and thus hinder the crack growth. There is thus wide scatter in crack growth for this type of material compared to materials containing a lower or higher fraction of whis- Fig. 2. Crack growth (Ac) versus temperature difference (An)for kers. Increasing the volume fraction of whiskers above representatives of the Ti(C, N) whisker composites 10 vol. leads to a continuously decreasing standard deviation in crack growth after thermal shock 4. Conclusion Alumina composites reinforced with different volume fractions of Ti(C, N) whiskers were produced to evaluate △T the thermal shock properties and to correlate the ther mal shock resistance with the amount of reinforcing phase. An indentation-quench test was used for the thermal shock measurements The amount of Ti(C, N) whiskers was varied between 051015202530354045 5 and 40 vol. of the total composite volume. The indentation fracture toughness increased from 2. 6 MPa Ti(C, N)whiskers(vol % m /2 for pure alumina to 5.0 MPa m /2 for the sample Fig 3. Levels of different percentage of crack growth for the whisker with 30 vol. TI(C N whiskers. The hardness als reinforced composites. It is evident from the figure that the best ther increased, from 17.6 to 24.2 GPa. The fracture tough mal shock resistance is reached at a fraction of 30 vol. whiskers ness and the thermal shock resistance increases with increasing fraction of reinforcing phase up to 30 vol% that was found to be the optimum volume fraction, so a 30 voL. clear correlation between the thermal shock resistance fracture toughness could be observed. The composites also show a clear R-curve behaviour Acknowledgements This work has been performed within the Inorganic Interfacial Engineering Centre, supported by the Swed ish National Board for Industrial and Technical Devel 3501732m023 opment (NUTEK) and the following industrial partners: Erasteel Kloster AB, Ericsson Cables AB, Crack length(um) Hoganas Ab, Kanthal AB oFCon AB Sandvik AB. Fig. 4. The R-curve behaviour for two of the whisker composites. Seco Tools ab and uniloc aB
composites are given in Fig. 2. The thermal shock resistance increases with increasing amount of whiskers up to a volume fraction of 30%, where the optimum thermal shock properties are observed. A further increase in the volume fraction of whiskers leads to a decrease in the thermal shock resistance, see Fig. 3. The volume fraction of whiskers giving the best thermal shock resistance (30 vol.%) is also found in commercially available ceramiccutting tools based on SiC-whisker reinforced alumina. The measurements show that the thermal shock resistance improves with increasing fracture toughness. Such a relation has also been observed for other brittle materials.10 A clear R-curve behaviour is observed for the Ti(C,N) whisker-reinforced composites, see Fig. 4. Comparing the error bars in Fig. 2, it is clear that the scatter is largest at about 10 vol.% whiskers. This can be explained by that such a material have properties intermediate between pure alumina and a whisker-reinforced composite. With only 10 vol.% whiskers present, some of the cracks will grow solely in the alumina matrix, without meeting any whisker, while some cracks will meet whiskers that absorb crack energy and thus hinder the crack growth. There is thus wide scatter in crack growth for this type of material compared to materials containing a lower or higher fraction of whiskers. Increasing the volume fraction of whiskers above 10 vol.% leads to a continuously decreasing standard deviation in crack growth after thermal shock. 4. Conclusion Alumina composites reinforced with different volume fractions of Ti(C,N) whiskers were produced to evaluate the thermal shock properties and to correlate the thermal shock resistance with the amount of reinforcing phase. An indentation–quench test was used for the thermal shock measurements. The amount of Ti(C,N) whiskers was varied between 5 and 40 vol.% of the total composite volume. The indentation fracture toughness increased from 2.6 MPa m1=2 for pure alumina to 5.0 MPa m1/2 for the sample with 30 vol.% Ti(C,N) whiskers. The hardness also increased, from 17.6 to 24.2 GPa. The fracture toughness and the thermal shock resistance increases with increasing fraction of reinforcing phase up to 30 vol.% that was found to be the optimum volume fraction, so a clear correlation between the thermal shock resistance and the fracture toughness could be observed. The composites also show a clear R-curve behaviour. Acknowledgements This work has been performed within the Inorganic Interfacial Engineering Centre, supported by the Swedish National Board for Industrial and Technical Development (NUTEK) and the following industrial partners: Erasteel Kloster AB, Ericsson Cables AB, Ho¨gana¨s AB, Kanthal AB, OFCON AB, Sandvik AB, Seco Tools AB and Uniroc AB. Fig. 3. Levels of different percentage of crack growth for the whisker reinforced composites. It is evident from the figure that the best thermal shock resistance is reached at a fraction of 30 vol.% whiskers. Fig. 2. Crack growth (c) versus temperature difference (T) for representatives of the Ti(C,N) whisker composites. Fig. 4. The R-curve behaviour for two of the whisker composites. 312 P. Pettersson, M. Johnsson / Journal of the European Ceramic Society 23 (2003) 309–313
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