Tribological Properties of Copper Molybdate Powder Solid Lubricants under High Temperature Conditions

Abstract

In our previous research, it was suggested that a sort of copper molybdate Cu 3 Mo 2 O 9 generated on the friction track of aluminum bronze coated with MoO 3 powder reduced friction under high temperature conditions. In order to study the lubricity of copper molybdate as high temperature lubricant, the lubrication properties of two kinds of copper molybdate powders, CuMoO 4 and Cu 3 Mo 2 O 9, were studied by comparing with the lubricity of CuO and MoO 3 powders. These powders were supplied to the sliding surface of the stainless steel specimens as solid lubricant and the friction test was conducted at various ambient temperatures ranging from room temperature to 700°C. Cu 3 Mo 2 O 9 and CuMoO 4 powders were synthesized by heating the mixture of CuO and MoO 3 powders in their own specific heating conditions and identified by XRD. Both of Cu 3 Mo 2 O 9 and CuMoO 4 powders showed lower friction coefficient and smaller wear amount of specimen with increasing ambient temperature. On the other hand, CuO and MoO 3 powders showed poor lubricating ability and high wear amount of specimens comparing with two kinds of copper molybdate powders. It was suggested that the enhanced maintaining abilities of copper molybdate on the stainless steel substrate at high temperature and generation of unoxidized copper under high temperature conditions could enhance lubricating abilities.

Tribological behaviour of self-lubricating materials at high temperatures

“…Compounds like silver molybdates may form in-situ at HT from low-temperature solid lubricants by means of tribo-oxidation [24], leading to improved tribological behaviour. However, this approach can be unsuitable for applications of interest involving thermal cycling like metal forming, as the ternary oxides formed at HT may not be lubricious at lower temperatures [128]. As a result of this, the reversal of the self-lubricating material to its original form at RT is one of the main challenges in ternary oxide lubrication [24], with no clear solution being reported in the existing literature.…”

“…This was attributed to the formation of molybdenum oxide compounds such as MoO 3 and CuMoO 4 already at RT, although no further testing at HT was reported. The behaviour of copper molybdates at HT has been studied by Takeichi et al [128]. In this case, Cu 3 Mo 2 O 9 and CuMoO 4 powder coatings were deposited and compared to CuO and MoO 3 using a ring-on-disc tribometer up to temperatures of 700°C.…”

“…The behaviour of copper molybdates at HT has been studied by Takeichi et al. [128]. In this case, Cu 3 Mo 2 O 9 and CuMoO 4 powder coatings were deposited and compared to CuO and MoO 3 using a ring-on-disc tribometer up to temperatures of 700°C.…”

“…It must also be noted here that the in-situ formation of ternary oxides at HT is favoured, as they may show inadequate tribological behaviour at lower temperatures when deposited as a coating [128]. In this context, the combination of silver and molybdenum compounds like MoS 2 has been extensively studied, as both compounds can provide effective lubrication at lower temperatures while reacting to form beneficial silver molybdates at HT.…”