Surface engineering is a rapid developing and large demanding technique
In the progress of surface engineering, it was found that due to every surface technique has its own advantages and disadvantages, single surface treatments technique usually cannot meet the requirements in the tribological design of harsh working condition, which boost the development of duplex surface engineering, defined in that two or even more techniques are used at the same time so as to have their characteristics mutually supplemented. There is no doubt that the duplex surface engineering is an innovative product in the period of development of surface engineering.
According to a research (T Bell,1998), many advances in the field of surface engineering has grown up in the past ten years. There are several different aspects, such as optimisation of traditional processes (e.g. electro- and electroless plating, weld surfacing, thermal spraying and thermochemical treatments), commercialisation of the modern techniques (e.g. CVD, PVD, plasma thermos-chemical processes, plasma spraying and ion implantation), developments of innovative hybrid technologies (e.g. plasma immersion ion implantation and plasma source ion implantation) and emergence of new coating materials (e.g. diamond and diamond-like coatings). Despite these existing surface technologies have been designed enough practical and been used with enhanced performance, the requirements of servicing in a different working situation until the emergence of duplex surface engineering has been achieved. An example of a new approach of duplex surface engineering for Ti alloys will be discussed in this essay.
2. Issues of the critical component of Ti alloy industrial application
In the development of aircraft industry, the driving force is largely dependent on the weight-loss rate. Normally, the use of titanium alloys with light-weight characteristics has become an important means of reducing the weight. In this aspect, the potential applications of Ti alloy could be related to shafts, gears and bearings. However, all of these applications involve unavoidable issues which are mechanical joints, fluctuating loads and tribological performance, especially the fretting in combination with fracture loads. Obviously, these factors do limit and have the impact on the driving force of aircraft.
3. exploration of the solution
In a previous research (S Bhowmick,2003), the researchers attempted to coat the titanium components with hard, low friction and thin films, such as PVD (physical vapour deposition) deposited TiN. However, this approach was not successful because such films could not bear high concentrated loads. On the basis of stress filed analysis, the thin and brittle coatings could deposit on relatively soft substrates such as titanium, thus the deformation usually started from the coating or substrate interface would be the most normal fracture under the high concentrated loads. Fortunately, this type of deformation can be solved by increasing the thickness of the film or the strength of the substrate material. Additionally, the bearing ability of loads in the system also will be improved.
As for above-mentioned issues, there are two solution ideas from a previous research (Leyland A,1994), one is treating the titanium alloy substrate by the chemical diffusion approach, and then processing the deposition of hard thin films (duplex treatments). Another one is considered about the multilayer thin coatings based on the different materials (e.g. TiN, TiCN, CrN OR CrC) or assorted micro-structures. However, these two approaches are not able to achieve easily, the limiting factors, such as the rare thickness of such layers, long processing time and economic issues, are still unavoidable resistance.
The present essay is going to discuss an enhanced idea to conquer the above-mentioned limitation. The concentrated point of this research (Valente T,2000) aims to develop the technique of surface treatment and design an innovative duplex surface system. The system consists of two parts that first step is using reactive plasma spraying (RPS) of a thick (approximately hundreds of micrometres) Ti/TiN composite film to deposit on the titanium based substrate, and that second step is adopting physical vapour deposition (PVD) of a thin (about a few micrometres) and hard TiN based coating (single or multilayer) to deposit on the thick coating made in the first step.
4. Design of the duplex surface system for Ti alloys
To achieve the design of the complex duplex surface system for Ti alloys, it is necessary that several aspects of using the reactive plasma spraying technique to deposit the thick interlayer should be considered. There is the first consideration whether the deposited Ti/TiN composite coatings (Ti nitrides dispersed in a Ti alloy) have a pretty chemical compatibility with the top layer and the substrate or not. The second important point is that the possibility to modify the mechanical properties of the coating by controlling the reaction parameters, such as deposition temperature, plasma gases and chamber pressure. Additionally, how to process an elastic modulus of the interlayer in an intermediate range between the substrate and the top layer should be considered as an improved aspect. The final point which is economic cost, such as time, workers or devices has to be considered. Obviously, the reactive plasma spraying technology is relatively competitive with other approaches of deposition in the point of economical view, because RPS technique has a duration of process, which means it only take few minutes for the deposition of hundreds micrometres. Thus, the combination of the RPS and PVD can provide beneficial cost ratio, especially when compares with other duplex system.
Basically, the proposed coating system can enhance the situation that the deposition of Ti/TiN interlayer are able to confine the deformation of the system, as well as decreasing the rate of change in the properties of the substrate or coating interface. As the top layer, the low friction TiN coating can not only increase the wear resistance, but also decrease the shear stress of interface as well as the possibility of debonding.