Carbon films - Basic knowledge, film types and properties
|Publication Date:||1 June 2012|
|ICS Code (Other treatments and coatings):||25.220.99|
The subject of the present VDI Guideline is carbon films which are deposited by the vacuum deposition method (PVD or CVD). These modern coating systems extend from extremely hard diamond films at one end of the scale, passing through a wide variety of mostly hydrogenated amorphous carbon films before reaching soft graphite films at the other end. This guideline pursues two aims: firstly, it should provide a uniform classification and nomenclature for carbon films; secondly, it should enable the prospective user of coated workpieces to make a preliminary selection of suitable carbon film types.
The present guideline is primarily intended for those considering using coatings for tribological loading cases and provides this user group with basic technical knowledge about carbon films. Thus it supplies the required information to allow the user to unambiguously identify the carbon films which are available on the market and also to compare them. For this purpose the guideline contains a classification of all carbon films which subdivides all known film types on the basis of their physical and chemical nature. This means that in discussions with a coating company a potential user will be able to identify the precise type of a specific coating being offered him or the types of the individual layers in case of coating systems. When presenting their products, coating companies are also recommended to provide alongside the commercial name the corresponding designation according to this classification. In case of amorphous carbon films, suppliers should refrain from providing only the generic term DLC (diamondlike carbon). Instead the specific film type should be indicated - for example, "a-C:H:Me" or "ta-C".
The guideline also includes a section with descriptions of the important properties of the individual film types. This should allow the user to pick the most suitable film type for his particular tribological application.
The carbon films which are listed in the classificatory section of this guideline are taken to mean those coatings in which carbon is the predominant constituent part and which are deposited by the PVD or CVD process. This group includes coatings consisting of graphite and diamond (the two crystalline variants of carbon), amorphous carbon films, and also plasma polymer films. In case of the latter two classes, the coating sometimes also contains not inconsiderable proportions of other elements, such as hydrogen, for example.
That part of the guideline dealing with characteristic coating properties (Section 6) includes diamond films and amorphous carbon films. It does not cover graphite films nor plasma polymer films. The latter are so diverse that an exhaustive treatment of them is simply not possible within the bounds of this guideline. The graphite films on the other hand are not very widespread in industrial applications and their inclusion therefore would not serve any useful purpose.
Essentially this guideline deals with those coatings which are produced on an industrial scale. This means we must, for example, exclude a-C:X and ta- C:X films (X = Si, O, F, N, B, and so on) which are yet under industrial development or which currently have only a very limited application.
Also not included are industrial barely-used graphene layers - single atom graphite layers for electronic applications - as well as carbon nanotubes (CNTs) that can only be deposited by processes using PVD (physical vapour deposition) and CVD (chemical vapor deposition) and are not used as coatings but individually or in bulk, e.g. as fillers for composite materials.
In the applications we will take into consideration all known tribological application cases for the carbon films dealt with - in other words, those cases where wear protection and/or friction are of prime importance. This means that both tool coatings and component coatings will be included. Carbon films are used for other purposes as well, such as in optical applications, for example.
All data provided in this guideline relate to typically encountered forms of the film types. Real coatings may vary from these on an individual basis since the range of parameters which affect coating properties is very wide. Furthermore, the details provided relate to single-layer systems, unless stated otherwise. In fact, amorphous carbon films frequently have multilayer systems. Their properties may differ from those of the single-layer systems.
We shall not be concerned either with intermediate layers (such as adhesion promotion coatings, for example) which are often deposited before the amorphous carbon films. Furthermore, the information applies only to first coatings. Stripping and reapplication of carbon films is possible in principle but may have an adverse effect on performance in comparison with a first coating. The same applies to overcoating - in other words, applying a fresh coating without stripping the previous coating first.
Paints and sprayed-on thermal coatings are not to be included amongst carbon films: although they can consist predominantly of carbon, they are not deposited by the PVD or CVD processes. Nor will we be dealing with surface modification methods. These include processes whereby carbon comes into contact with the substrate - as is case with hardening, for example. In this case, however, the carbon is not deposited but rather diffuses into the substrate surface and thus cannot correctly be regarded as a coating.
Carbon films in the broader sense can also include films which contain compounds of carbon and nitrogen - the so-called CNx films. This guideline will not deal with them either, but for relevant literature, see [1 to 3].
The specific application of CVD diamond coatings for tools is described in the series of guidelines VDI 2841.