DIAMOND LIKE CARBON COATING ?
Diamond Like Carbon Coatings? Carbon-based coatings are of wide interest due to their application in machine elements subjected to continuous contact where fluid lubricant films are exposed to heat and causing abrasion wear. This information below describes the tribological performance under dry conditions of duplex layered of the coating sequentially deposited by microwave excited plasma enhanced chemical vapor deposition on AISI 52100 steel.
The architecture of the coating comprised Cr, WC, (a-C:H) with a total thickness of 2.8 たm and compressive residual stress very close to 1 GPA. Surface hardness was approximately 22 GPA and its reduced elastic modulus around 180 GPA. Scratch tests indicated a well adhered coating achieving a critical load of 80 N. The effect of normal load on the friction and wear behavior’s were investigated with steel pins sliding against the actual coating under dry conditions at room temperature (20± 2 °C) and 35 – 50% RH.
The results show that coefficient of friction of the coating
decreased from 0.21 to 0.13 values with the increase in
the applied loads (10 - 50 N). Specific wear rates of the
surface coating also decrease with the increase in the
same range of applied loads. Maximum and minimum
values were 14 x 10-8 and 5.5 x 10-8 mm-3 /N m,
respectively. Through Raman spectroscopy and electron
microscopy it was confirmed the carbon-carbon contact,
due to the tribo layer formation on the wear scars of the
coating and pin. In order to further corroborate the
experimental observations regarding the graphitization
behavior, the existing mathematical relationships to
determine the graphitization temperature of the coating
steel contact as well as the flash temperature were used.
The quality and functional characteristics of man
engineering applications is determined by superimposed
mechanical loads and particular requirements on their
surface, such as wear resistance or low levels of
coefficient of friction (COF). The use of thin films as the
non-stoichiometric hydrogenated amorphous carbon
and has become widespread for the improvement of performance/life of steel components, in which specific properties at particular locations are needed without compromising the bulk material strengths.
Furthermore, the Anisotropic Global Spec coatings are also being used as solid lubricants in the industrial field to improve
tribological behaviors of machine components under very clean atmospheres or where fluid lubricants are not permitted. The deposition of the coatings is commonly obtained by plasma decomposition of a hydrocarbon-rich atmosphere at low substrate temperatures and high deposition rates.
The latter is a major advantage for most steels because the annealing temperature is not reached (<200 °C), therefore substrate hardness is not affected. In spite of the outstanding properties of the coatings, the COF and wear mechanisms depend significantly on the deposition method/parameters, variation in environment, either in vacuum or room atmosphere, relative humidity the substrate and counter body materials, the various coating architectures such as multiple or gradient layers, and very importantly, the wearing conditions, i.e. under dry, water, oil or a combination of these In the case of unlubricated conditions, the influence of loading and sliding velocity on the COF and wear rates becomes essential. Sharma, put forward variations in the COF and wear for different loads, under ambient and dry conditions.
The COF decrease with the increase of normal load for short sliding distances, and for long sliding distances, the COF decrease with the decrease of normal load. Upon the steady state there seems to be a convergence in the COF values ranging from 0.05 to 0.2. Meanwhile, wear rates increased with the increase of load. Similar trends were also identified in similar, previous research in contrast, Mo reported very little influence of the normal load on the COF of a WC/C coating tested under reciprocating sliding against pure titanium.
The COF values found were 0.13 upon the steady state. Therefore, the correlations between tribological performances of a particular DLC and operational conditions have a considerable importance for any possible application for certain machine components. Nevertheless, research has mainly focused on the modification of the physical properties of the new Diamond coatings and their evaluations from the tribological and environmental points of view On the whole, the studies on friction and wear of WC/H-DLC ́s for different applied contact pressures and under dry conditions are scarce.
The number of techniques to deposit hard coating films, plasma enhanced chemical vapor deposition process (PECVD) is extensively utilized due to the nature of the hydrocarbon gas used as the coatings incorporate more sp2 and H bonds which make the films softer than the majority of tetrahedral ta-C counterparts? [2, 18]. Furthermore, reactive magnetron sputtering is a relatively new technology that combines magnetron sputtering and PECVD to increase hardness and provide better wear resistance and low friction values in the coated steels.
In order to elude the detrimental environment effect of the mating surfaces and to carry off the heat generated through friction, lubrication processes have been successfully used and are designed to influence the friction and wear behavior of surfaces in contact. However, it is well known that hydrogenated coatings do not react with various lubricants, mainly oil and additives, because of their low surface energy and the lack of attracting polar groups from such oils and additives In this regard, much research has focused on the synergistic action between carbon- based coatings and new environment-friendly lubricants (lower S and P contents), in the boundary lubrication regime, but it is apparent that the tribological behaviors of the coatings under lubricated conditions are still indistinct.
Therefore, since the reactivity of the oil varies with friction surface, the combination of a tribo- coating operating under a lubricated condition is still an aspect under investigation. Thus, while the coefficient of friction has been found higher for the combination of steel/steel compared to the AC:H/steel combination, there are some cases where the former combination is lower than the latter.
It is clear that with the introduction of the diamond coatings in existing engineering systems, the main objective for their effective application is to ensure a consistent performance under dry and oil lubricated conditions. In this context, some of the important studies have been carried out with quite controversial outcomes and/or are not practical for industrial applications because the election of normal loads and sliding velocities are away from real engineering conditions.