Advanced Mechanical Surface Testing

Characterization of hard coatings – Part 1: DLC Coatings

Introduction

Diamond like coatings (DLC) are today one of the most often used coatings for improvement of mechanical frictional and tribological performance of many components and parts [1,2]. The diamond like carbon term comprises different types of coatings or films whose structure is formed by amorphous carbon. The main types of DLCs are

  • hydrogen free diamond like carbon (commonly referred to as a-C),
  • tetrahedral amorphous carbon (ta-C),
  • hydrogenated tetrahedral amorphous carbon (ta-C:H).

Included as DLC coatings are also amorphous carbon films that contain small amounts of dopants such as metals. The DLC coatings are usually deposited either by Physical Vapor Deposition (PVD) or by Chemical Vapor Deposition (CVD), in some cases with plasma enhancement (PECVD). The typical thickness of the DLC films is in the range of several micrometers although some DLC films can be as thin as several tens of nanometers [3].

This application report summarizes the use of indentation, scratch, tribology and coating thickness measurements for a complete characterization of mechanical properties, adhesion and thickness of DLC coatings.

Hardness and elastic modulus: indentation

The Anton Paar Table Top Nanoindentation Tester (TTX-NHT3) has been developed for testing hard thin films such as TiN, AlTiN, CrN or DLC. The TTX-NHT3 is an excellent tool for both research and quality control to characterize hardness and elastic modulus of DLC films. Besides hardness and elastic modulus, the TTX-NHT3 can also calculate elastic and plastic work. All these results are automatically calculated according to the ISO 14577 standard. Figure 1 shows indentation load-indentation depth curves obtained on sample with 2 μm thick DLC coating deposited on metallic substrate using the TTX-NHT3.

 

Suggested test parameters:

  • TTX-NHT3 (force range 0.1 to 500 mN)
  • Berkovich indenter
  • Fmax 2 mN to 100 mN
  • Loading rate 4 to 200 mN/min
  • Hold at Fmax: 5 s

The great advantage of the top reference in the NHT3 is not only its superior thermal stability but also the protection of the diamond indenter against damage. The NHT3 is therefore perfectly suitable for routine measurements in quality control or for university courses.

 

 

 

 

 

 

 

Figure 1 – Typical indentation curves on 2 μm thick DLC coating (maximum load 25 mN, Berkovich indenter).

 

 

 

 

 

 

 

 

Figure 2 – The Anton Paar Table Top Nanoindentation Tester (TTX-NHT3).

The indentation parameters have to be adjusted according to the thickness of the coating, surface roughness and hardness of the coating. It is recommended that the indentation depth should not exceed 10 % of the thickness of the coating (more detailed analysis of indentation conditions can be provided using our partner’s software). At the same time, the indentation depth should be at least 20 times higher than the surface roughness (Ra). In practice, a compromise is sometimes necessary; higher scatter of data has to be compensated for by larger number of automatic measurements in a matrix.

Coating adhesion: scratch testing

The scratch technique is used for determination of adhesion of components or substrates with surface coating. The method has been developed specifically for testing of adhesion of hard coat­ings and the DLC coating is therefore one the main applications of the scratch test technique. The adhesion of the coating is de­termined via critical loads, which are defined as the normal force corresponding to the first, second or third type of failure of the coating-substrate system. The scratch test can be used for both quality control as well as for research purposes.

  • The Anton Paar scratch testers provide patented feature of recording a synchronized Panorama image. The Pano­rama feature consists of recording the image of the entire scratch track and its synchronization with the recorded sig­nals. This patented feature allows fast and efficient com­parison of scratches on different samples even after tested samples are no longer present. Besides more reliable adhe­sion analysis, the Panorama feature is a great tool for archiv­ing of the scratch results.
  • Another important feature of Anton Paar scratch testers is the capability to measure the true penetration depth via the pre-scan and post-scan procedure. Knowing the true pen­etration depth, one can quickly see whether the scratch test has gone through the whole thickness of the coating to the substrate and thus if delamination occurred.

The following figures 3 to 5 show typical results of adhesion test­ing of DLC coatings.

The recommended parameters for a Revetest scratch tester are:

  • Diamond Rockwell indenter with 200 μm radius
  • Progressive load scratch mode
  • Scratch length 2 mm
  • Scratch load 0.5 N – 60 N.
  • Loading rate 300 N/min

 

 

 

 

 

 

 

Figure 3 – Typical scratch recording (penetration depth, residual depth, acoustic emission, coefficient of friction and normal force on a 2 μm thick DLC coating on metal.

 

 

Figure 4 – Panorama image of the scratch on the DLC coating.

 

 

 

 

Figure 5 – Images of the critical load areas on the DLC coating: a) first critical load Lc1 – side cracks, b) second critical load Lc2 – partial delami­nation, c) third critical load Lc3 – full delamination..

figure 6

Figure 6 – The RST3 scratch tester with microscope for adhesion testing of hard coatings.

The Anton Paar RST3 scratch tester is a robust instrument with maximum load of 200 N dedicated for testing DLCs and other types of hard coatings. It is the successor of the successful RST scratch tester which is widely recognized as a standard instru­ment in adhesion testing of hard coatings.

Coefficient of friction and wear: tribology

Tribometer devices such as pin-on-disk tribometer determine the friction and wear of two surfaces during sliding contact. Knowing the coefficient of friction (CoF) is important either for quality control of the DLC film’s production or for development of new DLC coatings with tailored coefficient of friction. Using the tribometer the coefficient of friction can easily be measured under different conditions (load, environment, sliding speed, test duration, temperature, etc.). Should the wear resistance and wear rate be important, the profile of the wear track generated during the tribological test can be analyzed using a profilometer. Once the cross sectional area of the wear track is known, the wear rate of the material can be calculated.

The Anton Paar tribometers provide fast and efficient determination of tribological properties under various well-defined conditions. Therefore they are indispensable when testing the DLC coatings for their frictional properties. Aside from CoF and wear rate determination, the tribological experiment can also be used to determine the life time of the coating under given conditions: at the moment of failure and after it, the CoF will change as the counterbody is not sliding against the DLC film but against the substrate. The CoF between the counterbody on the substrate is usually different from that of the DLC on the counterbody. The corresponding change can be observed on the CoF versus time (distance) curve recorded by the tribometer.

An example of a tribometer measurement on 3 μm thick DLC coating on metallic substrate is shown in Figure 7. The recommended measurement parameters for tribometer testing are:

  • Tribometer in rotation or reciprocating mode
  • Linear speed 10 cm/s
  • Normal load 5 N
  • Distance 1000 m

 

 

 

 

Figure 7 – Coefficient of friction as a function of distance measured on 3 μm thick DLC coating on metallic substrate. The measurement was performed at 340°C using the 450°C heating stage on the tribometer in linear reciprocating mode

 

 

 

 

 

Figure 8 – Profile of the wear track after pin on disk test at 300°C.

 

 

 

 

 

 

 

 

 

Figure 9 – The Anton Paar tribometer in the rotation mode. According to the application, the tribometer can quickly be transformed in linear reciprocating mode.

 

The great advantage of Anton Paar tribometer is that they can be easily transformed from the rotation mode to the linear reciprocating mode in order to simulate as closely as possible the given application. There are also numerous accessories such as liquid container, heating stage and profilometry for wear rate determination.

Coating thickness: Calotest

For many types of coatings, their thickness tolerances must be carefully respected as it can have catastrophic impact on the final application. The DLC is no exception and its thickness has to be monitored since larger thickness often leads to the generation of high residual stresses, which can result in the delamination of the coating.

The Anton Paar Calotest is a simple but very efficient device to measure the thickness of coatings by rotating ball with known diameter in order to create a crater. It is perfectly suited for coating thickness measurement during the manufacturing process since it is considered as ‘non-destructive’ (only very small calotte is usually created on the sample). The Anton Paar Calotest can store up to 10 protocols with predefined measurement parameters, which allows for easy testing of various production parts.

Since thickness plays an important role in both the indentation and scratch parameters, the Calotest is particularly useful to properly setup those tests. Figure 10 shows an example of a crater for a DLC coating on steel substrate. The calotte was done using 40 mm diameter steel ball, at 400 rpm and for 15 seconds.

The average thickness of the coating (1.0 μm±0.7 μm) was calculated automatically after a series of three measurements. during the measurements.

 

 

 

 

 

 

 

 

 

Figure 10 – Calotest on 1 μm thick DLC coating on steel substrate.

 

 

 

 

 

 

 

 

 

Figure 11 – The Anton Paar Calotest is compact yet very efficient tool for non-destructive measurements of coating thickness.

 

The Anton Paar Calotest can be provided with a microscope and Video software, which allows automatic generation of measurement reports in PDF, including images of the craters along with the calculated thickness of the layer(s).

Summary

This application report demonstrates the capability of Anton Paar instruments for a complete characterization of DLC coatings. The methods of instrumented indentation, scratch testing, tribology and thickness measurements are necessary tools to provide such a complete characterization of mechanical properties of DLC coatings. The Anton Paar Table Top Nanoindentation Tester can be advantageously used for hardness and elastic modulus measurements whereas the Revetest Scratch Tester determines the adhesion of the coatings via progressive load scratch tests. Since the frictional properties of the DLC coatings are of utmost importance in many applications, the pin-on-disk linear reciprocating or rotational mode of the tribometer is an indispensable tool for measurement of the coefficient of friction, wear rate and life time of these coatings. The Calotest measures the thickness of the DLC layers in a very fast and efficient way and provides critical information for the indentation and scratch measurements. With these four measurement methods the DLC coatings can be thoroughly characterized during quality control or development process.

 

 

References

1) S.V. Hainsworth, N.J. Uhure, Diamond like carbon coatings for tribology: production techniques, characterisation methods and applications, Int. Mater. Rev. 52 (2007) 153–174. doi:10.1179/174328007X160272.

2) G. Dearnaley, J.H. Arps, Biomedical applications of diamond-like carbon (DLC) coatings: A review, Surf. Coat. Technol. 200 (2005) 2518–2524.

3) S. Miyake, W. Kurosaka, K. Oshimoto, Nanometre scale mechanical properties of extremely thin diamond-like carbon films, Tribol. – Mater. Surf. Interfaces. 3 (2009) 158–164. doi:10.1179/175158309X12586382418535.