M. Hadad et al. /Wear 260(2006)634-641 and fracture toughness [15-18]. It has, however, been found that the improvement in mechanical properties of laminates does not lead to a higher wear resistance [5]. Furthermore, it has been shown that laminates with compressive stresses has higher wear resistance than in those of tensile stresses [14. For instance, in case of Si3N4, mechanical properties and abrasive wear resistance were reported to vary inversely to grain size of precipitated second phase like Bn or TiN [19]. Si3N4 was found to be unsuitable for machining of steel because of the chemical affinity between this pair of materials 2]. Studies on friction coefficient and wear of self-mated Si3N4 have been widely reported Under un-lubricated condi- (0.7-0.9)and high wear valu (5 x 10- to 10-mm/Nm) have been observed [20-22] Si3N4-nin composites under dry conditions show that wear resistance ofSi3 N4 was increased by addition of Tr ver stacking sequence of Si3N4/Si3N4+%TIN laminate system. However, under dry conditions, self-mated TiN has a slightly higher friction coefficient than self-mated SiN4. This has into dough. Tapes were then formed by twin roll compaction een attributed to the absence of titanium oxide formation, and were stacked in 4/1 system(a tensile layer Si3N4-X% which could reduce friction and wear rate [26]. In order TiN composites with 620 um of thickness and compres- to reveal if there is a tribological impact of the improved sive Si3N4 layer with 150 um of thickness as shown in mechanical properties of the laminate structure, the tribo- Fig. 1). The stacks were hot pressed in vacuum. Different logical behaviour of three materials: bulk Si3 N4, Si3N4-TIN residual stresses exist in the layers due to the difference in composites and Si3N4/Si3N4-TIN laminates have been inves- coefficient of thermal expansion(CTE)between two different tigated here. In particular, different sliding directions with components In the previous work, the mechanical proper- respect to the layers in laminates have been tested under oscil- ties were analysed [27-29]. The results are summarised in lating, un-lubricated sliding Table 2.2. Wear testing and analysis Friction and wear experiments were conducted with a 2.1. Specimens preparation reciprocating movement in a ball-on-block sliding wear est setup Is The specimens used in this investigation were hot pressed 95 un-lubricated wear testing. The top oscillating speci Si3N4, Si3N4-TiN composites and Si3N4 based laminates. men( ball) acts on the bottom block(tested specimen)at For composite materials, TIN was introduced during the pre-programmed frequency of oscillation, stroke and load milling of starting powders in order to insure homogeneous settings. The friction force is continually measured by a sen- dispersion of TIN into the Si3N4 resulting in composite mate- sor. The upper oscillating specimen is a 9.525 mm diameter rials with 10, 20 and 30 wt%TiN. Multi-layer laminate mate- bearing ball of silicon nitride having Ra=0.007 um surface rials consist of Si3 N4 bulk and Si3 N4-X%TIN composite roughness, -1500 HV hardness and a density of 3.2 g/cm layers. The layers of laminate were manufactured by milling Test conditions were as follows: 1.3 GPa of Hertzian pressure the powders, addition of a binder and drying of the mixture stroke length of 2 mm, reciprocating frequency of 10 Hz, rel- Table I Mechanical property of bulk, composites and laminates materials according to 27-29 Material Toughness KiC(MPam) Strength(MPa Young,s modulus(GE Hardness(Hv 0.5) 790.2 Si3Na-10% TN 4.4 1384 Si3Na-20% TnN Si3Na-30% TiN 4.7 784.7 Laminates Si3 Na/Si3 N4-10% TIN 13N4Si3 N4-20% TIN 9.24 Si3N4/Si3 Na-30% TINM. Hadad et al. / Wear 260 (2006) 634–641 635 and fracture toughness [15–18]. It has, however, been found that the improvement in mechanical properties of laminates does not lead to a higher wear resistance [5]. Furthermore, it has been shown that laminates with compressive stresses has higher wear resistance than in those of tensile stresses [14]. For instance, in case of Si3N4, mechanical properties and abrasive wear resistance were reported to vary inversely to grain size of precipitated second phase like BN or TiN [19]. Si3N4 was found to be unsuitable for machining of steel, because of the chemical affinity between this pair of materials [1,2]. Studies on friction coefficient and wear of self-mated Si3N4 have been widely reported. Under un-lubricated condi￾tions, high friction coefficients (0.7–0.9) and high wear values (5 × 10−5 to 10−4 mm3/N m) have been observed [20–22]. Si3N4–TiN composites under dry conditions show that wear resistance of Si3N4 was increased by addition of TiN [23–25]. However, under dry conditions, self-mated TiN has a slightly higher friction coefficient than self-mated Si3N4. This has been attributed to the absence of titanium oxide formation, which could reduce friction and wear rate [26]. In order to reveal if there is a tribological impact of the improved mechanical properties of the laminate structure, the tribo￾logical behaviour of three materials: bulk Si3N4, Si3N4–TiN composites and Si3N4/Si3N4–TiN laminates have been inves￾tigated here. In particular, different sliding directions with respect to the layers in laminates have been tested under oscil￾lating, un-lubricated sliding. 2. Experiments 2.1. Specimens preparation The specimens used in this investigation were hot pressed Si3N4, Si3N4–TiN composites and Si3N4 based laminates. For composite materials, TiN was introduced during the milling of starting powders in order to insure homogeneous dispersion of TiN into the Si3N4 resulting in composite mate￾rials with 10, 20 and 30 wt% TiN. Multi-layer laminate mate￾rials consist of Si3N4 bulk and Si3N4–X% TiN composite layers. The layers of laminate were manufactured by milling the powders, addition of a binder and drying of the mixture Fig. 1. Macrostructure of layer stacking sequence of the 4/1 Si3N4/Si3N4 + %TiN laminate system. into dough. Tapes were then formed by twin roll compaction and were stacked in 4/1 system (a tensile layer Si3N4–X% TiN composites with ∼620
m of thickness and compres￾sive Si3N4 layer with ∼150
m of thickness as shown in Fig. 1). The stacks were hot pressed in vacuum. Different residual stresses exist in the layers due to the difference in coefficient of thermal expansion (CTE) between two different components. In the previous work, the mechanical proper￾ties were analysed [27–29]. The results are summarised in Table 1. 2.2. Wear testing and analysis Friction and wear experiments were conducted with a reciprocating movement in a ball-on-block sliding wear configuration. The test setup is similar to ASTM G133- 95 un-lubricated wear testing. The top oscillating speci￾men (ball) acts on the bottom block (tested specimen) at pre-programmed frequency of oscillation, stroke and load settings. The friction force is continually measured by a sen￾sor. The upper oscillating specimen is a 9.525 mm diameter bearing ball of silicon nitride having Ra = 0.007
m surface roughness, ∼1500 HV hardness and a density of 3.2 g/cm3. Test conditions were as follows: 1.3 GPa of Hertzian pressure, stroke length of 2 mm, reciprocating frequency of 10 Hz, rel￾Table 1 Mechanical property of bulk, composites and laminates materials according to [27–29] Material Toughness KIC (MPa m1/2) Strength (MPa) Young’s modulus (GPa) Hardness (Hv 0.5) Bulk Si3N4 4.26 790.2 303 1556 Composites Si3N4–10% TiN 4.47 685 311 1384 Si3N4–20% TiN 4.61 883.9 317 1336 Si3N4–30% TiN 4.71 784.7 330 1396 Laminates Si3N4/Si3N4–10% TiN 9.75 590 308 – Si3N4/Si3N4–20% TiN 9.24 699 305 – Si3N4/Si3N4–30% TiN 14.3 707 313 –