Titanium tribology soars
Paul Stratton, Project Manager of The Linde Group in Sheffield, UK, describes work to enhance titanium gears for load bearing.
High strength titanium alloys, typically Ti-6Al-4V, are used extensively in aeroplanes. However, titanium has poor tribological properties which limit its application. Attempts to improve on this involve a thermochemical treatment that diffuses oxygen or nitrogen into the surface. But the treatment must be carried out above the α→β‚ transition temperature to produce sufficient case depth for general engineering applications within a reasonable time (say, 24 hours). Unfortunately, this generally degrades the core properties of the titanium.
Multi-element diffusion treatments could offer advantages, both in terms of treatment temperature and case properties.
Nitrogen and oxygen diffusion
To explore this possibility, Paul Stratton, Project Manager of The Linde Group, worked with colleagues at the Linde Laboratories in Unterschleissheim, Germany, carrying out diffusion treatments using a range of gases or mixtures of gases at temperatures below the α→β, transition temperature. Some of the combinations included nitrogen.
The team found that when samples are treated with nitrogen-containing mixtures, the treatment results are markedly inferior compared to mixtures containing no nitrogen. In addition, the layers treated with nitrogen-containing mixtures were brittle and tended to spall as they were prepared for microstructural examination. As a result, the researchers switched to argon as the carrier gas.
When using nascent nitrogen, derived from ammonia, a good diffusion layer formed, with a case depth to 550HV of 50µm and a near surface hardness of 757HV.
This layer has properties that are similar – but not identical – to those formed by diffusing in oxygen for the same time and at the same temperature (the depth of the layer formed by diffusing oxygen is 59µm). However, hardness close to the surface is similar at 816HV. Both oxygen and nitrogen stabilise the phase, which etches white in the micrographs).
Carbon and oxygen diffusion
When carbon and oxygen derived from either carbon dioxide (CO2) or carbon monoxide (CO) are diffused at the same time at 850ºC, a second lamella phase forms near the surface. This is more pronounced when carbon monoxide is used. After 24 hours treatment the diffusion zone is 72µm and the surface hardness is 922HV. This is deeper and harder than the properties resulting from either of the single element treatments using oxygen or nitrogen.
When treatments using CO are carried out over a range of times, the case depth achieved shows a normal Fick’s Law relationship. When the treatment temperature was increased (see graph, p37, top, left), the case depth rises as expected.
Another effect of the treatment temperature is structural coarsening. For example, there is significant coarsening of samples treated at 900ºC compared with those at 750ºC. It is not significant at processing temperatures of 850ºC and below for Ti-6Al-4V. This means that components are not heat-treated a second time to refine the core after diffusion treatment.
Wear testing has been carried out at Swerea IVF AB in Sweden. The pin-disk wear test uses hemispherical test pins wearing against a hardened tool steel disk. The load is high initially, equivalent to approximately four Newtons per square millimetre after 50m sliding, but decreases as wear proceeds and the scar gets larger. When the wear rate of CO treated Ti-6Al-4V is compared to that of optimally carburised 20MnCr5 tested under the same conditions, the wear is an order of magnitude less. The treated titanium is wear resistant enough for the disk material to adhere to the pin at high loads, limiting the test load.
The compared diffusion treatments using oxygen, CO and CO2 show that the CO treatment produces the lowest wear rate. The rate for CO-treated samples is half that of those that are oxygen-treated, while samples treated with CO2 are somewhere between the two. This correlates directly to the near surface hardness of the layers.
The thick layers formed by 24 hour treatment with CO are strong enough to withstand the effects of high loads that might deform the substrate when hard thin layers are used, making them effective for heavily loaded gears. Titanium gears manufactured from a suitable alloy and treated using CO should reduce the weight of a gear box by approximately 40%, taking into account the larger sections needed to carry the load.
Further information: BOC