TechArchive

Original article date: July 2000

Problems in hydraulic systems arising from abrasive and adhesive wear can be solved by PVD hard coatings, says Dr ANDREW BLOYCE of Balzers

Providing high power density, compact dimensions and outstanding controllability, hydraulic systems support a wide range of industrial applications. But the performance of hydraulic systems is often compromised by the tribological load bearing capacity of the conventional materials pairs used in hydraulics, such as nitrided steels and bronze.

Problems can also arise if fluids are contaminated with solid particles – these can cause abrasion and erosion, resulting in damaging leaks. Optimum filter configurations and regular maintenance cannot completely eradicate this problem. In addition to abrasive wear, the performance of hydraulic systems is also compromised by adhesive wear/seizure, which can cause failure, even in high performance pumps using clean oil.

These problems can now be solved by protecting hydraulic system components with physical vapour deposition (PVD) coatings. Significantly increasing the load bearing capacities and wear resistance of components, PVD coatings promote exceptional surface hardness and sliding characteristics. They have been used successfully in tooling applications for many years and have improved the reliability and cost-effectiveness of many machining applications.

The coatings are applied under high vacuum conditions by evaporation (TiN, TiCN, CrN) or by sputtering (WC/C). During these processes, the coating material is evaporated in plasma or sputtered from targets by ion bombardment and then deposited on the parts to be coated. This high vacuum process offers significant advantages over atmospheric thermal spraying techniques and gas/chemical bath nitriding and galvanising. The benefits include:

• Compound material composition, including amorphous carbon coatings (WC/C) which provide excellent friction behaviour combined with outstanding hardness

• High accuracy – PVD coatings are only a few thousandths of a millimetre thick. This ensures uniform profile coverage to eliminate finishing requirements

• Increased load capacity – high vacuum deposition prevents contamination and ensures a metallurgical bond with excellent layer adhesion.

Screw spindle pumps are exposed to both abrasive and adhesive wear when used to convey contaminated oil or grinding system coolants. The life of these pumps, however, can be significantly extended by applying a TiN coating to the spindles. Moreover, in applications involving water and oil, WC/C coatings provide effective protection against seizure between spindles and between the spindle and the casing.

Similarly, the life of a rotary vane pump with additive-free hydraulic fluid can be greatly increased if the vanes are PVD-coated. Tests have shown how WC/C coating is the most effective way of preventing seizure in the contact zones between the vanes and the stator ring. Nitrided pistons in axial piston pumps can often fail under very high pressure and speed conditions owing to seizure between piston and barrel. WC/C coatings dramatically increase the performance range and reliability of such pumps. In fact, identical tests involving coated and uncoated nitrided pistons have shown that the PVD coating process eliminates seizure problems which compromise the reliability of the uncoated pistons.

WC/C coatings also improve the performance of sliding show/cam plate systems. Contamination can cause abrasion of bronze sliding shoes, leading to the pitting and destruction of the pump. Offering significant cost savings by extending component life, WC/C coated steel sliding shoes are much less sensitive than the bronze components. Even after an exhaustive 1000 hour test, the shoes remained virtually unmarked.

• Balzers

July 2000