Coatings for better wear resistance and
for decorative purposes.
Many different industries make use of thin, hard coatings to improve
the properties of their products. To ensure that these properties
can be guaranteed, precise standard testing methods are needed.
This extensive study analysed and validated a range of test procedures
for different properties including adhesion, elasticity and thickness.
The results of the trans-European collaboration have been offered
to national and European testing bodies.
Harder, stronger, tougher, lighter and longer lasting. Coatings can help change and improve the properties of so many of the goods and machines we use today. Some protect from water damage, for example, while others reduce the wear and tear of moving parts. Manufacturers can also redesign their products with environmental protection in mind as lighter, less resource-intense goods can be made if they use thin hard protective coatings.
Producers choose coatings to enhance performance and also to reduce the frequency of repair or replacement. This "added value" and potential cost savings mean that coatings find their way on to many different products in a wide range of industries including chemicals, communications, medicine, transport, aerospace, optics, construction, engineering and microelectronics. What's more, according to the project coordinator, Dr Vetters of Germany's Institut für Werkstofftechnik, demand for coatings is growing dramatically.
When producers decide to use a coating, they want to be sure that it will exhibit certain properties. In other words, they want it to do its job and do it well. "There is an increasing need to control coating specific parameters such as their mechanical properties or the adhesion between coating and substrate," says Dr Vetters. Without such validated test methods, quality control becomes inhibited and trade can be disrupted. The consequences for the companies involved in producing or using coatings, many of which are SMEs, could be disastrous.
Making use of
The tests have to be used by many companies and testing laboratories. This means that the test results have to be meaningful to everyone using them and they must all be performed in the same way. "Devices for bulk characterisation are mostly inapplicable for testing thin films. Due to the development and fast progress in micro-mechanics on the one hand and the development of advanced components in traffic, space craft and communication on the other, manufacturing and component fabrication needs superior precision on the micro- and nano-scale of tolerances," says Dr Vetters.
With this in mind, 14 partners from industry, national testing laboratories and research centres brought together a variety of skills to develop and validate test methods for thin hard coatings. After determining the important parameters that affect the test and finding the best test conditions, each of the chosen tests was tested in a round-robin fashion, with 34 laboratories participating in all. With these results, the partners have presented test guidelines to the European Committee for Standardisation.
Detecting hidden defects or coating imperfections also requires comparative testing against standard reference samples. The project team chose a master alloy as a reference, the shiny golden titanium nitride. It has high hardness and useful thermo-mechanical properties. It is mainly used for industrial applications like drilling, cutting and for electrical switches. You may have seen it in a more decorative setting though. It is also used for glasses frames and jewellery as well as in some medical applications, including false teeth.
How thick is the coating?
Several different properties are important in the coatings world. The project partners looked into different tests to find the most precise and reliable method for measuring thickness, elasticity, abrasion resistance and adhesion.
Coating thickness can be measured by acoustic microscopy. Here, surface acoustic waves are generated in the sample surface. Measuring the change of velocity of these waves gives a measurement of thickness. In this procedure, the sample is not destroyed. This makes it a useful industrial test. The laboratories outlined this test method and it is now ready for standardisation.
They also calibrated the high frequency ultrasonic test equipment for coating thickness measurement. So that SMEs can use this information in their problem-solving strategies, the project partners will communicate these results more widely on an application-oriented basis.
Other properties are not as easy to measure non-destructively. Testing how well a coating adheres to the surface is performed by a scratch test where the sample is, as the name suggests, scratched with a diamond which is dragged across the surface while being pushed down with an increasing weight. Testing revealed that the most reliable way to determine "critical load" is to examine the damage under a microscope. Scratch test manufacturers will use this result to optimise their instruments, and the study results are being used to produce an improved European pre-standard on the test.
Coatings bounce back
A good coating will also "bounce back" after it has been indented. By directly measuring the coating hardness versus indent depth with a nano-indenter device, testers can determine the sample's elasticity. Alternatively, they can use impulse excitation of a thin sample bar which is subjected to a blow from an impact hammer. A microphone measures the frequency of the signal detected and from this, elastic constants are calculated. A good correlation was found between the elasticity data determined with these methods and acoustic microscopy results. So, the work shows that these three ways are well suited for determination of elasticity of thin coatings.
Abrasion seriously reduces the coating's effectiveness. Testing this property is complex and difficult. However, the consortium did develop two test procedures to be submitted as standards.
The success can be attributed to partners' backgrounds which encompass a range of interests and different cultural approaches. The study also benefited from its neutrality. This ensured that no single product specification was deemed more important than another. Finally, the participants are confident that company involvement helped both develop industrially useful techniques and exploit and spread the results through different sectors.