Effect of different surface treatments and coatings on the scuffing performance of hardened steel discs at very high sliding speeds

The paper describes the results of scuffing experiments using a high speed two disc rig, in which a range of different surface treatments and coatings was investigated. The rig used is capable of simulating the operation of heavily loaded gear contacts in terms of maximum Hertzian contact pressure (up to 1.7 GPa), temperature (100 °C oil feed), and sliding speed (up to 25 m/s). The reference scuffing performance was that of axially ground case-carburised or nitrided steel discs having a surface finish of 0.4 μm Ra. The different surface conditions were as follows. The ground discs were superfinished to less than 0.1 μm Ra using a proprietary polishing method. Two different super-hard coatings were investigated. The main conclusions to be drawn from the work are: nitrided surfaces are superior to case-carburised surfaces, but it is essential to remove the compound layer (‘white layer’) to achieve durability at high sliding speeds. The benefits of superfinishing are clearly demonstrated; this improves the scuffing performance and also gives lower friction and bulk temperatures. Hard coatings show promise, and the triple combination of nitriding, superfinishing and hard coating gave particularly impressive scuffing resistance in these tests.     

Effect of reciprocating wire slurry sawing on surface quality and mechanical strength of as-cut solar silicon wafers

This paper investigates reciprocating wire slurry sawing for photovoltaic (PV) silicon wafering and compares the resulting wafer surface quality and its mechanical strength to that obtained in unidirectional wire sawing. It is found that wire reciprocation creates two significantly different morphological or cutting zones on the wafer surface. The zone width varies with the distance travelled by the wires, the cutting location on the wafer surface, and direction of wire motion. The size of the morphological zone created during forward motion of the wire is larger than that created during its backward motion. The zone width is found to decrease along the wire cut direction. In addition, there appears to be greater kerf loss and increased surface roughness in the forward cutting zone. In general, results suggest a higher material removal rate during forward motion of the wire than during backward motion. Notwithstanding the surface morphology variations, the fracture strengths of reciprocating wire sawn wafers are found to be quite similar to that exhibited by wafers produced by unidirectional wire sawing.