M. Hadad et al. /Wear 260(2006)634-641 637 by mechanical contact and subsequent particle detachment ature and high relative humidity, tribochemical reactions are that takes place at the micrometer and submicrometer scale known to be the dominate wear mechanisms [22]. Titanium The residual stresses in laminates did not contribute to any oxide was shown to form under mild conditions(up to 0.9 GPa improvement of wear resistance compared to the composites. of Herzian pressure and 0. 1 m/s of velocity)[26, 30]. The The residual stresses in layers of laminate were calculated won surface of Si3 N4 bulk tested under the ambient temper and reported in the previous work [29]. A possible explana- ature(Fig 3a) looks like a polished area containing porosity tion is that local contact pressures in particle abrasion are However, the worn surface of composites(Fig. 3b) shows much higher than residual stresses. Similar results of lami- cavities in the order of the grain size of Tin(bright areas) nates structured materials have been published, where resid- This may result from intergranular fracture at the interfaces ual stresses have not contributed to improve wear resistance Si3 N4/TIN, and subsequent pull-out of Tin particles. Bulk and composites generate very smooth wear debris, agglom erated and attached to each other Fig 4). The wear debris has 3.1.4. Wear rate of materials at 300C both spherical and irregular shape between 0. 1 and 0.5 um of An increase in temperature to 300C led to a decrease of diameter. The loose wear debris and the worn surfaces were 10 times in wear resistance of Si3 N4-30% TiN composites. analysed by EDs. We could not detect the presence of oxy- Similarly, the wear resistance of Si3 N4 bulk decreased by a gen. Therefore, we conclude that the Tin O2n-I film removal factor five times. However, wear resistance of Si3N4-30% is not the main wear mechanism These results are in agree- TIN composites still was two times higher than bulk Si3N4 ment with the literature [25] wear resistance under the same conditions The worn surface of laminate in Fig. 3c shows the two inner layers, Si3N4 and Si3N4-30% TIN composite(longitu- 3. 2. Wear mechanism and friction dinal sliding). The composite inner layer within the laminate shows a worn surface similar to worn composites. Concerning Mechanical and/or tribochemical were the dominant wear the Si3N4 inner layers, the polished area with porosity could mechanisms of Si3 N4 based ceramics[30]. Mechanical wear not be evidenced The coefficient of friction has been reported modes include intergranular fracture, delamination and grain in many reviews to vary between 0.85 and 1 [22, 30].Our boundary fatigue at the interface between B-Si3N4 and glass experimental results of all materials at the ambient tempera phase on and below the surface. Furthermore, the elongation ture revealed high friction values from I to 1.2 (Figs. 5 and 6) of these cracks below the surface may lead to the delamina- This may be due to the absence or removal of the tribo-layer ion of larger surface region. The tribochemical reactions of like titanium oxide because of the high contact pressure. This Si3 na based ceramic mainly leads to the formation of phases is coherent with the absence of oxygen in the contact area. such as SiO2, SiOH and SiO,N, [20]. Si3 N4-TiN composites The observed wear debris was repulsed out of contact area by are able to form a stable oxide film(Tin O2n-i), which can be mechanical push. Otherwise the wear debris could have had beneficial to the tribological behaviour. Under high temper- a bearing role as a third body, which could reduce friction Bulk inner layer Composite 30%iN inner tayer aces at ambient temperature: (a)Si3N4 bulk, (b)Si3N4-30% nin composite worn surface inclined some degree and(c) on(not inclined)M. Hadad et al. / Wear 260 (2006) 634–641 637 by mechanical contact and subsequent particle detachment that takes place at the micrometer and submicrometer scale. The residual stresses in laminates did not contribute to any improvement of wear resistance compared to the composites. The residual stresses in layers of laminate were calculated and reported in the previous work [29]. A possible explana￾tion is that local contact pressures in particle abrasion are much higher than residual stresses. Similar results of lami￾nates structured materials have been published, where resid￾ual stresses have not contributed to improve wear resistance [5]. 3.1.4. Wear rate of materials at 300 ◦C An increase in temperature to 300 ◦C led to a decrease of 10 times in wear resistance of Si3N4–30% TiN composites. Similarly, the wear resistance of Si3N4 bulk decreased by a factor five times. However, wear resistance of Si3N4–30% TiN composites still was two times higher than bulk Si3N4 wear resistance under the same conditions. 3.2. Wear mechanism and friction Mechanical and/or tribochemical were the dominant wear mechanisms of Si3N4 based ceramics [30]. Mechanical wear modes include intergranular fracture, delamination and grain boundary fatigue at the interface between -Si3N4 and glass phase on and below the surface. Furthermore, the elongation of these cracks below the surface may lead to the delamina￾tion of larger surface region. The tribochemical reactions of Si3N4 based ceramic mainly leads to the formation of phases such as SiO2, SiOH and SiOxNy [20]. Si3N4–TiN composites are able to form a stable oxide film (TinO2n−1), which can be beneficial to the tribological behaviour. Under high temper￾ature and high relative humidity, tribochemical reactions are known to be the dominate wear mechanisms [22]. Titanium oxide was shown to form under mild conditions (up to 0.9 GPa of Herzian pressure and 0.1 m/s of velocity) [26,30]. The worn surface of Si3N4 bulk tested under the ambient temper￾ature (Fig. 3a) looks like a polished area containing porosity. However, the worn surface of composites (Fig. 3b) shows cavities in the order of the grain size of TiN (bright areas). This may result from intergranular fracture at the interfaces Si3N4/TiN, and subsequent pull-out of TiN particles. Bulk and composites generate very smooth wear debris, agglom￾erated and attached to each other (Fig. 4). The wear debris has both spherical and irregular shape between 0.1 and 0.5
m of diameter. The loose wear debris and the worn surfaces were analysed by EDS. We could not detect the presence of oxy￾gen. Therefore, we conclude that the TinO2n−1 film removal is not the main wear mechanism. These results are in agree￾ment with the literature [25]. The worn surface of laminate in Fig. 3c shows the two inner layers, Si3N4 and Si3N4–30% TiN composite (longitu￾dinal sliding). The composite inner layer within the laminate shows a worn surface similar to worn composites. Concerning the Si3N4 inner layers, the polished area with porosity could not be evidenced. The coefficient of friction has been reported in many reviews to vary between 0.85 and 1 [22,30]. Our experimental results of all materials at the ambient tempera￾ture revealed high friction values from 1 to 1.2 (Figs. 5 and 6). This may be due to the absence or removal of the tribo-layer like titanium oxide because of the high contact pressure. This is coherent with the absence of oxygen in the contact area. The observed wear debris was repulsed out of contact area by mechanical push. Otherwise the wear debris could have had a bearing role as a third body, which could reduce friction. Fig. 3. SEM micrographs of worn surfaces at ambient temperature: (a) Si3N4 bulk, (b) Si3N4–30% TiN composite worn surface inclined some degree and (c) laminates at longitudinal sliding direction (not inclined).