Hidden potential of stainless steel: The role of low-temperature surface hardening before DLC coating in improving tribological performance

Wendel Michael - Bodycote Specialist Technologies GmbH (Germany)

Components in automotive and transportation applications are often subjected to high alternating stresses, temperature fluctuations, and abrasive conditions, which dramatically challenge the material’s durability and performance. The demand for components that deliver highly specialized performance under extreme conditions is constantly increasing. The growing complexity of these requirements can often exceed the capabilities of a single enhancement.

As every process naturally has its limits, one way to overcome these limitations is to combine processes with the aim of accumulating positive properties and eliminating undesirable ones. Therefore, the beneficial effects of combining low-temperature surface hardening processes, such as S³P featuring Kolsterising® (below 500°C), with subsequent DLC coatings applied via plasma-assisted vacuum processes on austenitic stainless steel AISI 316L were investigated. Laboratory tests to evaluate the mutual positive influences of both technologies included point-loading impact tests, abrasive-wear resistance tests, and scratch tests with increasing load.

This research determined that combining the mentioned processes significantly reduces typical issues associated with hard coatings, such as flaking, delamination, or the so-called eggshell effect. A prior surface hardening process provides high hardness, a smooth hardness transition from the soft stainless steel base material, and better substrate support. The DLC coating, in turn, achieves extremely high surface hardness, which is a major advantage in abrasive applications and environments where maintaining constant coefficient-of-friction values is important.

The application possibilities of this dual treatment extend to parts for liquid pump- and piping components, fuel injectors and electromagnetic actuators, where wear resistance and long-term reliability are crucial for overall system performance.

In conclusion, the combination of low-temperature surface hardening technology S³P Kolsterising® and DLC coatings on stainless steels offers an advantage over the individual treatments alone. It holds particular promise for critical components exposed to demanding conditions, providing enhanced longevity and operational reliability in conventional, hydrogen-based, or electrified vehicles.