Diamond-like coatings (known universally as DLC) are a type of coating that has really taken the racing world by storm. There are a whole range of coatings that come under the description of DLC, with varying levels of hardness and coefficient of friction. They are also very thin.
They are not though something that can be applied without due thought to any situation that the designer feels may merit such a coating. The geometry of the part and the underlying material must also be considered when selecting the correct coating. The stiffness and strength of the underlying part have an important impact on the likely success of DLC, and there are a wide range of applications where we can expect DLC not to work.
Where the problem to be solved is one of excess wear, a hard, thin, low-friction coating may prove to be ideal. However, if the wear problem is one caused by subsurface stresses exceeding the material's limitations, putting something black and shiny on the surface is only going to make it look pretty initially; it will do very little to alter the basic problem apart from changing the subsurface stress field owing to lower friction. In situations where full hydrodynamic lubrication generally prevails, DLC may provide only limited protection against damage at start-up.
In situations where the substrate material is insufficiently stiff - either through its modulus, or by the design of the part - the DLC may simply crack as the substrate deforms elastically. Where sharp edges run across a surface, problems are especially likely. There are other situations where the adhesion of DLC can be insufficient to prevent flaking of the coating.
So, is there anything that can be done in such situations? Unsurprisingly, the answer is yes. Our friendly coatings suppliers have developed solutions for many of the situations that might previously have rendered DLC unsuitable.
A number of suppliers have developed multilayer coatings that are aimed at solving the problems above. In terms of adhesion, a 'base layer' is selected which has sufficient adhesion to the substrate, and to which the DLC adheres adequately. Such processes are similar to techniques used in plating, where a thin base coating improves adhesion. Sometimes more than one different type of coating is used for the base layer, common ones being metallic titanium, chromium nitride (CrN), titanium carbide (TiC), titanium nitride (TiN) and titanium carbo-nitride (TiCN). An example of a multiple layer coating is one that is applied to a titanium substrate: TiN may be applied first, followed by TiCN and then finally the DLC.
Such multi-layer coatings, (also known as functionally graded coatings) can be used to prevent brittle flaking of DLC by providing a stiffer substrate, thereby reducing compliance to a level that the limited ductility of the DLC coating can cope with.
With such a huge array of different coatings and coating systems, one should not expect the designer to know which coating should be applied in every situation. Thankfully coatings companies are usually very helpful in recommending a suitable coating for a given application.
Fig. 1 - Multilayer coatings are commonly applied to cam followers
Written by Wayne Ward