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with typical adhesive wear features evidenced by numerous adhesive craters and deep ploughing grooves. Clearly, untreated titanium can not be used under rubbing conditions ~ven with oil lubrication and a low sliding to rolling ratio.
In contrast, the wear rate of the TO treated specimen (1.57x1 0·3
mg m·') was dramatically reduced by more than 2
orders of magnitude over the untreated material and it was even lower than that of the hardened steel counterpart (1 .61 x1 O"'mg m·') by a factor of more than 10.
Scuffing is a form of severe sliding wear characterised by unacceptably high friction and a high degree of surface damage, which is associated with local solid-state welding between the rubbing surfaces under high duty operating conditions. The anti-scuffing capacities of untreated and TO treated material were evaluated by obtaining the critical load-to-failure during the stepwise loading process of oil lubricated sliding wear tests
The untreated material, sliding against a hardened En19 steel wheel under oil lubrication, showed a strong tendency to scuffing. On the other hand, the TO treated material (with oxide) had an excellent anti -scuffing capacity, involving three orders of magnitude improvement over that of the as-received material.
This desired anti-scuffing capacity conferred by TO treatment could be ascribed to the favourable lubrication condition promoted by the surface oxide layer. This is largely because the surface rutile layer possesses a high ionic factor, which promotes high wettability of polar lubricants, thus enhancing lubrication conditions.
PALLADIUM TREATED THERMAL OXIDATION PROCESS
Another simple and effective surface modificat ion technique, namely palladium-treated thermal oxidation (PTO), has been developed in the present research towards recently for corrosion resistant titanium designer surfaces.
Both TO treated and PTO treated CP Ti exhibit superior corrosion resistance to untreated material either in immersion testing or in electrochemical testing, although the PTO treated material is significantly superior. It is believed that the excellent corrosion resistance of both the TO and PTO treated titanium in boiling HCI solutions mainly results from the protective surface oxide layers formed during the TO and PTO treatments because they are essentially inert and resistant in this environment. The significantly increased lifetime for the protective surface layer breakdown of the PTO treated material over the TO treated material may be attributed to the following factors: (1) the beneficial effect of palladium on the formation of protective surface layers by modifying the structure, morphology and composition of the layers and/or (2) the existence of palladium close to the coating/substrate interface, which may increase the passivating ability of pinholes and improve coating adhesion at the layer/substrate interface.
• Tribological characterisation of TO and PTO treated titanium
specimens also revealed that PTO treated titanium and its alloys gives higher critical load over TO treated materials. Thus titanium designer surfaces with a synergistic combination of both high tribological performance and elevated corrosion resistance can be achieved.
OXIDATION BOOST DIFFUSION PROCESS
Although both TO and PTO processes have proved to be effective surface engineering techniques for enhancing the tribological as well as corrosion properties of titanium and its alloys, the load bearing capacity of either TO or PTO treated titanium alloys is not high enough to withstand the high stresses encountered in such general engineering components as bearings and gears, and thus deep case hardening is necessary. It is not possible to increase the depth of the surface modified layer by further use of high temperatures and/or long times without causing severe stratification or scaling. Consequently, a new oxygen boost diffusion (OD) process has been developed for deep case hardening of titanium and its alloys. However, although OD treated titanium alloys exhibited significantly improved abrasive resistance over untreated materials, their sliding wear resistance was only marginally increased. This is presumably due to the fact that OD can increase hardness via oxygen interstitial solid solution hardening, but metallurgical compatibility would be hardly changed. Consequently, a novel duplex system combining low friction, high wear resistance diamond-like coating (DLC) with OD deep case hardening has been designed.
A total hardened case of about 300 IJm, twenty times that of the hardened layer produced during TO or PTO treatment, was successfully achieved following the optimised OD treatment. Amorphous hydrogenated carbon a-C:H or DLC containing a small amount of titanium was deposited on the OD treated Ti6AI4V using a rf-reactive sputtering system. To overcome the main problem encountered with the deposition of DLC at low temperature, i.e. poor coating adhesion to the substrate, a graded layer, Ti/TiN/TiCN/TiC, between the substrate and the DLC layer was used.
The outstanding stress-resisting performance can be attributed to the optimised design of the duplex system. The compositionally graded intermediate layer, Ti/TiN/TiCN/TiC, eliminates interfacial cracking and leads to a homogenisation of the stress distribution in the coating under local loading, thus high adhesion is achieved.
Therefore, a duplex treatment is essential in achieving a high load bearing capacity. In summary, this novel duplex system can effectively extend the service conditions for titanium components in terms of a high sliding ratio and I or higher loads, and thus it could be an important step towards titanium designer surfaces.
These processes, together with a full list of references, are described in detail in Issue 6, Volume 50, 'Industrial Lubrication and Tribology'.
David Margaroni 110 I 0 • ffiJIU I QQQ
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