The effect of oxidation on the room-temperature mechanical properties of four different silicon

nitride ceramics is investigated. The strength and the elastic modulus of the whole material and

the hardness, the fracture toughness and the elastic modulus of the internal region (without

surface oxide layer) are determined for the materials oxidized at temperatures up to 1400°C. It

is found that the fracture toughness decreases during oxidation at 1000°C. Reduction of the

elastic modulus of the materials oxidized at 1400°C is also observed. A part of the changes in

the mechanical properties is interpreted by the changes in the phase composition

Arató, Péter

Gubicza, Jenő

Wéber, Ferenc

English

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OXIDATION-INDUCED CHANGES IN MECHANICAL PROPERTIES OF SILICONNITRIDE CERAMICSJ. Gubicza, P. Arató and F. WéberResearch Institute for Technical Physics and Materials Science, H-1525 Budapest P.O.Box 49,HungaryThe effect of oxidation on the room-temperature mechanical properties of four different siliconnitride ceramics is investigated. The strength and the elastic modulus of the whole material andthe hardness, the fracture toughness and the elastic modulus of the internal region (withoutsurface oxide layer) are determined for the materials oxidized at temperatures up to 1400°C. Itis found that the fracture toughness decreases during oxidation at 1000°C. Reduction of theelastic modulus of the materials oxidized at 1400°C is also observed. A part of the changes inthe mechanical properties is interpreted by the changes in the phase composition.1. INTRODUCTIONSilicon nitride ceramics are generally used in high temperature oxidizing environments.During oxidation of these ceramics a strongly oxidized layer is formed on the surface1,2. This layerhas a great influence on the oxidation resistance and the room-temperature mechanical propertiesof silicon nitride. It was established by numerous authors that the strength decreased duringoxidation because of the large defects in the surface oxide layer3,4. Although this scale protects theinternal region of the material from the strong oxidation, the chemical and the phase compositionsof the internal part change during the oxidation heat-treatment1,5. These changes in thecompositions may induce changes in the mechanical properties of the internal part.In this paper the oxidation-induced changes in the room-temperature mechanical properties ofthe whole sample (strength, elastic modulus) and those of the internal part (hardness, fracturetoughness and elastic modulus) are investigated for different silicon nitride materials oxidized attemperatures up to 1400°C.2. EXPERIMENTAL PROCEDURESMixtures of Si3N4, AlN, Al2O3 and Y2O3 powders were milled in ethanol in a planetary typealumina ball mill. The compositions of the four starting powder mixtures are given in Table 1. Thesamples were compacted by dry pressing at 220 MPa. The materials were sintered in high puritynitrogen by a two-step sinter-HIP method using BN embedding powder. The dimensions of as-sintered specimens were approximately 3.5 mm x 5 mm x 50 mm. The samples were groundbefore oxidation. Oxidation took place in static air at 800qC, 1000qC, 1150qC, 1300qC and 1400qC for up to 48h. The oxidation resistance of the samples was characterized by the weight-gain andthe density-loss data. The apparent density of the as-sintered and the oxidized materials wasmeasured by the Archimedes method.TABLE 1The compositions of the starting powder mixtures of the materials investigated.Composition (wt%)Material Si3N4 AlN Al2O3 Y2O3X 90.9 0 3 6.1A 90 0 4 6B 87 4 4 5F 69 15 12 4The Young’s modulus and the strength of the samples were determined before and after theoxidation by a room temperature four point bending test with spans of 40 and 20mm. The hardnessand the indentation fracture toughness of the internal region were determined before and afteroxidation by a Vickers diamond indenter using a load of 100 N. Before the indentationmeasurement a wide surface region of the sample was removed by grinding and the new surfacewas carefully polished. The fracture toughness was calculated by the formula of Niihara et al.6because Palmqvist cracks were observed at the corners of the Vickers pattern for all the materialsinvestigated. The elastic modulus of the internal part is significantly different from that of thewhole sample oxidized at 1300°C and 1400°C because at these temperatures the porous surfaceoxide layer is relatively thick as compared with those formed at lower temperatures. The Young’smodulus of the internal region of these materials was measured by depth sensing indentation (DSI)method. The loading rate and the maximum load were 14.4 mN/s and 500 mN, respectively. Forthe explanation of the oxidation induced changes in the mechanical properties, the crystallinephases formed in the materials were identified by X-ray diffraction (XRD).3. RESULTS AND DISCUSSIONThe oxidation behaviour of our materials was characterized by the weight-gain and thedensity-loss data. The values of these quantities were different from zero only after oxidation at1300°C and 1400°C (Table 2). The decrease of the apparent density during oxidation is aconsequence of the increasing porosity in the material. Significant difference in the oxidationresistance of the materials investigated can be observed after oxidation at 1400°C. As can be seenin Table 2, material F is the most oxidation resistant material among the four ceramics. Theoxidation rate of material X is higher than that of material F. Materials A and B are the leastoxidation resistant materials and the oxidation rate of B is a bit higher than that of A.TABLE 2The weight-gain ('m/A) and the density-loss ('U) data for the materials oxidized at 1300°C and1400°C.1300°C 1400°CMaterial 'm/A (mg/cm2) 'U (%) 'm/A (mg/cm2) 'U (%)X 0.49 1.6 1.34 4.7A 0.47 1.6 4.04 8.0B 0.59 1.1 4.68 9.3F 0.47 0.3 0.69 1.3The Vickers hardness (HV) numbers of the internal regions of the as-sintered and the oxidizedmaterials can be seen in Figure 1. The standard deviations of the hardness values are less than 0.3GPa. The hardness of the materials investigated does not change significantly during oxidation,except for material B oxidized at 1300°C, the hardness of which is higher than that in the as-sintered state. This increase of hardness can be attributed to the change in the phase compositionduring oxidation as follows. The XRD analysis showed that E-SiAlON (Si6-zAlzOzN8-z) was themajor phase in each as-sintered material. It was observed that the z value of E-SiAlON increasedwith the increase of aluminium and oxygen contents of the starting powder mixture (z=0 formaterials X and A, z=0.5 for material B and z=1.0 for material F). The z value changed duringoxidation only in material B at 1300°C where a large amount of E-SiAlON phase with high z value(z=0.8) was formed beside the major E-SiAlON (z=0.5). This phase formation may cause theincrease of hardness of material B. This idea is supported by the observation that material F, inwhich the major E-SiAlON phase has also high z value (z=1.0), has the highest hardness amongthe four as-sintered materials.510152014.915.714.115.414.914.9material X14000C13000C11500C10000C8000Cas-sinteredHV [GPa]510152014.114.514.914.915.114.3material A14000C13000C11500C10000C8000Cas-sinteredHV [GPa]510152015.113.816.214.515.114.3material B14000C13000C11500C10000C8000Cas-sinteredHV [GPa]510152016.416.916.016.515.816.4material F14000C13000C11500C10000C8000Cas-sinteredHV [GPa]FIGURE 1The Vickers hardness of the internal region of the as-sintered materials and those oxidized atdifferent temperatures.The indentation fracture toughness data of the internal part of the as-sintered materials andthose of the materials oxidized at different temperatures are shown in Figure 2. The standarddeviations of the toughness values are less than 0.2 MPam1/2. The most significant change in KIC isits decrease after oxidation at 1000°C for all the materials investigated. This reduction oftoughness can be explained by the crystallization of the intergranular phase. Crystalline minorphases were not detected in the as-sintered state as well as after oxidation at 800°C. At 1000°CY2SiAlO5N (B-phase) in materials B and F, E-Y2Si2O7 and Y2SiAlO5N in material X and E-Y2Si2O7 in material A were formed as crystalline intergranular phases. As a consequence of thiscrystallization the density of the intergranular phase increases and the volume of it decreases4,5,7.The latter can cause development of tensile stresses at the grain boundaries4. These stresses mayresult in easy crack propagation at the grain boundaries and lower fracture toughness of thematerials oxidized at 1000°C. Although crystalline intergranular phases were also observed afteroxidation at temperatures higher than 1000°C, such a significant decrease of the toughness was notdetected probably because of annealing of the stresses developed due to crystallization4.4565.8material X5.66.05.85.25.914000C13000C11500C10000C8000Cas-sinteredKIC[MPam1/2]4565.35.45.75.05.65.9material A14000C13000C11500C10000C8000Cas-sinteredKIC[MPam1/2]4565.45.75.55.74.85.7material B14000C13000C11500C10000C8000Cas-sinteredKIC[MPam1/2]4564.84.94.84.55.15.4material F14000C13000C11500C10000C8000Cas-sinteredKIC[MPam1/2]FIGURE 2The fracture toughness of the internal part of the materials in the as-sintered state and afteroxidation at different temperatures.The effect of oxidation at different temperatures on the four point bending strength isillustrated in Figure 3. The standard deviation of the strength values is 11%. The strength decreaseswith the increase of the oxidizing temperature for all the compositions. At the temperatures below1000°C the strength degradation can be explained by the decrease of the fracture toughness. Athigher temperatures the large defects (e.g. cavities, cracks) in the strongly oxidized surface scalecan cause the reduction of the strength3,4. As the thicker oxide layer probably contains largerdefects, the higher strength degradation occurs in the material having thicker surface oxide scale. Ithas been found that the thicker oxide layer is linked to the higher density-loss after oxidation8.Consequently, the higher the reduction of the strength is, the higher the density-loss is afteroxidation at 1400°C (see Figure 3 and Table 2).200300400500600341411491533563606 material X14000C13000C11500C10000C8000Cas-sinteredStrength [MPa]200300400500600700348433575558631675 material A14000C13000C11500C10000C8000Cas-sinteredStrength [MPa]100200300400500450253266436467490 material B14000C13000C11500C10000C8000Cas-sinteredStrength [MPa]100200300400500303334360373377456 material F14000C13000C11500C10000C8000Cas-sinteredStrength [MPa]FIGURE 3The four point bending strength of the as-sintered and the oxidized materials.The effect of the oxidation on the Young’s modulus of the whole material and that of theinternal part is shown in Figure 4. The standard deviations of the elastic moduli are less than 10GPa. The elastic modulus of the whole sample decreases significantly during oxidation at 1400°Cfor all materials. The role of the low modulus of the porous oxide layer in this reduction of elasticmodulus has been discussed elsewhere9. The oxidation at 1400°C results in significant internalmodulus reduction too for all the materials investigated. The reason for this decrease of moduluscan not be the increase of the internal porosity because the hardness as well as the modulus oughtto have decreased significantly with increasing porosity10,11. This internal modulus reduction maybe caused by the change in the chemical composition of the intergranular phase during oxidation.150200250300191208267281273257260M aterial X14000C13000C11500C10000C8000Cas sinteredYoung's modulus [GPa]150200250300213248182252276268263268Material A14000C13000C11500C10000C8000Cas sinteredYoung's modulus [GPa]100150200250300234269149284249268269M aterial B14000C13000C11500C10000C8000Cas sinteredYoung's modulus [GPa]150200250300229248259247242249Material F14000C13000C11500C10000C8000Cas sinteredYoung's modulus [GPa]FIGURE 4The influence of oxidation on Young’s modulus of materials investigated. The shaded and theopen columns represent the moduli of the whole sample and the internal part, respectively.4. SUMMARYThe oxidation-induced changes in the mechanical properties of the whole sample and theinternal region of silicon nitride ceramics with four different compositions were studied.The hardness changed significantly only during oxidation of material B at 1300°C. Theincrease of the hardness was probably due to the increase of the amount of the major E-SiAlONphase with high aluminium and oxygen contents (z=0.8). This idea was supported by theobservation that material F, in which the major E-SiAlON phase had also high z value (z=1.0), hadthe highest hardness among the four as-sintered materials.The KIC of each material was significantly lower after oxidation at 1000°C than it had beenbefore. This reduction of toughness was probably due to the internal tensile stresses developed atthe grain boundaries because of the crystallization of the intergranular phase. At highertemperatures these stresses are assumed to be annealed because the deterioration of the toughnesswas not observed.The strength decreased with increase of the oxidizing temperature for all the compositions. Atthe temperatures below 1000°C the strength degradation may be mainly due to the decrease of thefracture toughness. Above 1000°C the reduction of the strength is thought to be a consequence ofthe increase of the defect size in the surface oxide layer.A significant reduction of the internal Young’s modulus was observed for all the materialsafter oxidation at 1400°C. This was caused by the low modulus of the porous surface scale and thedecrease of the modulus of the internal region. The latter might have been caused by the change inthe composition of the intergranular phase.ACKNOWLEDGEMENTThis work was supported by the Hungarian National Scientific Fund in contract numbers T-025479 and F-025478.REFERENCES1) A. Rendtel, G. Grathwohl and F. Tummler, cfi/Ber. DKG 69 (1992) 1992) C. O’Meara and J. Sjöberg, J. Am. Ceram. Soc. 80 (1997) 14913) Y. Ukyo, J. Mat. Sci. 32 (1997) 54834) Ch. He, L. Wang and J. Wu, J. Mat. Sci. 28 (1993) 48295) M. K. Cinibulk and H.-J. Kleebe, J. Mat. Sci.28 (1993) 57756) K. Niihara, R. Morena and D. P. H. Hasselman, J. Mat. Sci. Lett. 1 (1982) 137) T. Ekström and M. Nygren, J. Am. Ceram. Soc. 75 (1992) 2598) J. Gubicza, P. Arató, F. Wéber and A. Juhász, J. Mat. Sci. Letters, 17 (1998), to be submitted9) J. Gubicza, P. Arató, F. Wéber and A. Juhász, J. Mat. Sci. Letters, 17 (1998), in press10) P. Arató, E. Besenyei, A. Kele and F. Wéber, J. Mat. Sci. 30 (1995) 186311) F. Tancret, G. Desgardin and F. Osterstock, Phil. Mag. A, 75 (1997) 505

Oxidation-induced changes in mechanical properties of silicon nitride ceramics

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