Tribology Letters 8 (2000) 219–222 219 Effect of densification and TiB 2 reinforcement on the wear of molybdenum disilicide Sheela K. Ramasesha a , Srikari Tantri P. a , E. Moses Jayasingh a and S.K. Biswas b a Materials Science Division, National Aerospace Laboratories, Bangalore, India b Department of Mechanical Engineering, Indian Institute of Science, Bangalore, India Received 5 April 2000; accepted 9 August 2000 Wear tests were done in a pin-on-disc machine by sliding MoSi 2 pins against hard-steel discs in a normal load range of 5–140 N and a speed of 0.5 m/s under nominally dry conditions in the ambient. The specific wear rate of the pin undergoes two transitions: severe to mild at low load and mild to severe at high load. The mild-wear domain is distinguished by the formation of a protective mechanically mixed layer of steel and its oxides, transferred from the counterface in particulate form. Increasing the hardness by densification and TiB 2 reinforcement lowers the specific wear rate and expands the mild-wear load domain. However, even when the volume wear rate is normalised with respect to the real contact area (load/hardness) the non-dimensional wear factor is still seen to decrease with densification and reinforcement. This indicates that fracture toughness may also play an important role in determining the wear-resistance of these materials. The surface coverage on the pin by the mechanically mixed layer increases with densification and reinforcement. Keywords: wear, molybdenum disilicide, densification, reinforcement, hardness, fracture toughness 1. Introduction Molybdenum disilicide, an intermetallic compound, has high wear resistance, especially in aggressive chemical en- vironments [1]. Its dry tribology is comparable to that of oxide ceramics [2,3]. This is attributed to the material’s high strength, moderate density, high thermal conductivity and chemical inertness at high temperature. However, it has low fracture toughness at temperatures below 1000 ◦ C. Further, low-density (porous) MoSi 2 is known to crumble at 500 ◦ C due to severe oxidation, when heated for a pe- riod of time. If MoSi 2 is considered for dry tribological applications, the above problems are likely to play detri- mental roles. In the present paper, we address these issues by (1) identifying the mechanisms of wear, (2) recording the effect of densification on wear, and (3) recording the effect of reinforcement by oxidation resistant TiB 2 on wear. The latter two treatments enhance material hardness. Thus, to isolate the effects of porosity and reinforcement we study wear in terms of a non-dimensional wear fac- tor W , which is obtained by normalising the experimental wear rate by the real contact area (normal load/hardness). The real contact area was found to be about three to four orders less than the apparent contact area. 2. Experimental Homogeneous mixtures of Mo and Si powders were hot pressed in a graphite die between temperatures of 1650 and 1800 ◦ C under 20–25 MPa pressure for 10–15 min to obtain samples of different densities (86, 93 and 97%). To produce composite compacts, TiB 2 powder (10–20 wt%) was added to the Mo and Si mixture and hot pressed. The samples were characterized from X-ray diffrac- tograms (XRD model Philips-PW 1730) and scanning elec- tron micrographs (LEO 4401) taken on the surface perpen- dicular to the hot-pressing direction. Vickers hardness of the bulk and surface hardness of the test specimens were measured with a load of 1 kg and contact time of 15 s (Matsuzawa, MXT-Digital Microhardness Tester, 7α). The sliding-wear tests were conducted on a pin-on-disc machine with a maximum loading capacity of 200 N and maximum speed of 5 m/s. The wear is measured by a lin- ear variable differential transformer (LVDT, working range ±2 mm and resolution ±1 μm) which records the pin dis- placement and friction by a load cell attached to the pin holder. The pins (12 mm diameter and 2.5–3.5 mm thick) were surface ground to 1 μm and slid on heat-treated EN- 24 steel discs (0.39% C, 0.22% Si, 1.43% Ni, 1.02% Cr, 0.22% Mo and 0.6% Mn) which were ground to a roughness of 0.2 μm. The hardness of the ground disc was 50 RC. The wear of the counterface was measured using a Taylor Hobson profilometer. Sliding against the pin did not gen- erate any deep overall crater, but wore out the tips of the disc asperities. The roughness (Ra) of the worn track in a direction normal to the sliding direction reduced from about 0.4–0.35 μm, when a monolithic pin was slid against the counterface at 50 N and 0.5 m/s for 30 min. This is about 0.01% of the linear wear of the pin. This was considered very small and the wear data presented here thus refer only to a pin wear. The wear tests were done on pins by varying the load from 5 to 140 N after running in for about 60 min to get  J.C. Baltzer AG, Science Publishers