Tough Coatings

Torsten Doering
Product Manager Components
Oerlikon Balzers
Coating USA Inc.
Amherst, N.Y.

Typical physical-vapor-deposition (PVD) hard coatings are 20 times thinner than a human hair, yet they can drastically improve performance, boost reliability, and extend service lives of tool and machine components. In addition to conventional PVD hard coatings such as TiN (titanium nitride), so called "tribological coatings" with optimized frictional properties have been developed in recent years. These coatings protect highly stressed components that see sliding or rolling contact. They handle a range of applications in motor-sport, fluid power, medical, and aerospace including engine, transmission, pump, motor, and bearing components.

The bulk material provides different properties than the PVD-coated surface. The primary role of the tribological coating is to reduce friction so components perform better and last longer. It's the combination of an appropriate substrate material, surface topography, and PVD hard coating that spells the difference between design success and failure.

For example, various tungsten carbide/carbon (WC/C) and diamondlike carbon (DLC)-coated valve train and transmission components used in racing motorcycles reportedly improve power output and torque throughout the entire rpm range. Measurements using a dynamometer, or dyno for short, have shown that ceramiclike DLC and Balinit C (WC/C) coatings boost engine power and torque by 2 and 3%, respectively, while protecting components from wear.

Less wear improves component life and ensures more consistent performance throughout a race or season. Moreover, these thin hard coatings can be applied to off-the-shelf components with no need to redesign the parts with which they mate. Less friction, lower operating temperatures, and more-consistent power can be expected.

Hard coatings also minimize adhesive wear between mating steel parts. Adhesive wear, also known as scoring, galling, or worse — case seizing, results when two solid surfaces slide over one another under pressure. Surface projections, or asperities, plastically deform and eventually weld together under the high localized pressure. As sliding continues, these bonds break. This creates cavities on one surface and projections on the other. Tiny abrasive particles can also form causing additional wear.

PISTON RINGS
Piston rings also perform better with engineered coatings. Engine maker Scania AB, in Sweden, tested different piston-ring coatings against gray-cast iron liners. This is a configuration common in motorsport, two-stroke, and other big engines.

The 6-hr test ran at a frequency of 10 Hz and put parts under an 8-MPa load at 175°F. The chrome-ceramic (CKS) and Balinit C coatings showed no significant wear on the piston-ring OD surfaces. But the wear on the liner was significantly different.

The hard and porous chrome-ceramic surface doesn't run in as well as the Balinit C with its low coefficient of friction (CoF). This boosted the wear on both the chrome-ceramic liner surface and the piston ring. Obviously, it is undesirable to wear mating surfaces, even if the coated component is protected. Another aspect is the total frictional loss or engine efficiency. The piston train is the biggest contributor of all engine systems to frictional loss with about 30%. Low-friction coatings can improve efficiency, longevity, and increase power output.

Another test done by a Japanese piston-ring company compared galvanic-chrome coating, nitriding, PVD-CrN, and PVDWC/C coated piston rings against a 12% silicon-aluminum alloy piston with oil as a lubricant. Here, the ring will press less against the cylinder liner if the system starts to fail. Leakage and power loss result. Of the coatings tested, the wear rate of WC/C was the lowest. This mainly comes from the coat-ing's low CoF and ability to thwart adhesive wear. The choice of coating depends on the liner material and other aspects of the tribological system.

BEARINGS, GEARS, AND TRANSMISSIONS
Unlike plain-journal bearings, wave-journal bearings have a wave profile circumscribed on the inner-bearing diameter. The wave amplitude is equal to a fraction of the bearing clearance and represents a breakthrough technology because of geometry that allows excellent load bearing capacity and thermal and dynamic stability.

Appropriate materials (hard-hard steel) ensure bearing geometry doesn't change over time. And Balinit C and DLC coatings improve bearing performance under start/stop and oil out conditions. In one test conducted by the University of Toledo at NASA Glenn Research Center in Cleveland, there was no bearing damage after 1,000 start/stop cycles and after subsequent 80-min test at 4,000 rpm under a 400-lb load and oil-starved conditions. PVD coatings make the wave bearings a viable solution for applications that see frequent starts and stops and in cases of accidental oil starvation. The same test with an uncoated bearing had to be stopped after 637 start/stop cycles with full lubrication because of excessive wear.

Design trends for transmissions include lightweight, more-efficient constructions that can carry heavier loads, consume less lubricant, use fewer additives, and have longer maintenance intervals. These requirements tend to increase wear. Alternative materials alone won't always meet these challenges.

Failure in gear transmissions depends greatly on load and peripheral speed. Sliding wear results from low speeds and lack of continuous lubricant film between tooth faces. This results in gear teeth making direct contact (boundary lubrication) with each other. Scuffing comes about at low speeds and higher loads or when lubricant film viscosity and thickness decreases and ultimately ruptures.

Likewise, surface fatigue or pitting takes place on tooth surfaces when the maximum number of load cycles or loading capacity is exceeded. As the teeth repeatedly roll over one another, fine cracks develop at grain boundaries or inclusions mostly beneath the surface where the pressure is the highest. The carbon-based PVD coating Balinit C (WC/C) is one candidate that has proven effective against all these wear mechanisms over the years. The coating reduces local surface pressures (Hertzian pressure) and improves reliability of poorly lubricated gears by separating the two metallic gear surfaces with a hard ceramiclike layer.

A test of a helicopter transmission under emergency conditions (loss of transmission fluid) revealed the capabilities of WC/C. Two identical transmissions were tested in a rig setup, one with coated gears, the other without. The transmission with uncoated gears failed after about 1 hr. This was shown by its rise in temperature. The time-before-failure according to the test profile was not enough to ground the helicopter safely. The transmission with coated gears ran for more than 6 hr, three landing cycles could be simulated and the transmission was still in working condition. The WC/C coating reduced friction and avoided scuffing.

Another example of a planetary gear for a concrete mixer also showed significantly less wear with coated gears. In this high-load/low-speed situation Balinit C was reportedly one of only a few coatings that boosted performance when two coated surfaces made contact. This has to do with its structure, composition, and run-in behavior.

HYDRAULIC PUMPS
Producers of hydraulic drives are confronted with lubrication and corrosion issues. The trend towards lighter weights and higher pressures and speeds means that hydraulic components must handle more severe tribological stresses. With coatings, it's possible to reduce abrasion, allow dry operation for pneumatic valves, and handle most acids and alkaloids safely.

Uncoated vanes in an accelerated vane-pump test (with additive-free hydraulic oil) reportedly scuff after a few minutes. Similar test results could be obtained with water and CFC-free refrigerants as a medium in pumps, valves, and motors. CrN coatings are a frequent choice for these applications. Different CrN coatings are available. Balinit CNI, for example, applies using a sputtering process with a maximum temperature of 480°F so a broad range of materials can be coated. It provides a smooth, hard surface with a 50% lower CoF than other thin, dense galvanic Cr coatings.

SURFACE PREP
Bonds between surfaces and coatings depend on surface conditions. Surfaces must be metallically bright, ground, honed, polished, or lapped. Brazed surfaces must be free of flux, lubricants, and other residues. Braze materials can't contain cadmium, lead, or other low-vapor-pressure alloy additions. Grinding cracks, burrs, oxide skins, and rehardening burns on surfaces must be avoided.

Blind holes and inside contours must be contamination free. Plugs and screws should be removed before cleaning and coating.

The white layer of EDM (electrodischarge machining) surfaces must be reduced by running several finish cuts and by microblasting before coating. For best results surface roughness should be less than the thickness of the coating. Typical surface roughness values for components are 16 in. (0.4 m) or better. Only oil-free polishing agents should be used before coating.

HEAT TREATMENT
Heat treatment before coating must be such that the coating temperature does not cause loss of hardness or dimensional change. Typical maximum coating process temperatures are 480 to 950°F (249 to 510°C) depending on the coating system. Ball-bearing steels, case-hardening steels, and special-purpose tool steels with proper heat treatment can be coated with carbon-based coatings.

Special low-temperature coatings are available. High-speed steels, higher tempering temperature tool steels, and stainless steels are coatable without restrictions. Nitrided surfaces can be coated after mechanical pretreatment, and plasma nitriding is the preferred method for subsequent coating.

General-purpose construction steels can be PVD coated but lack strength to support a hard coating. So they should serve only in low-load applications. Nickel and titanium are readily coatable. Polishing of these softer materials before coating must not introduce stresses or trap any type of residue in the surface. Copper and magnesium can be coated with restrictions to applicable temperature, cleaning process, and applied load they will see after coating.

Chrome and nickel-plated metals can be coated but adhesion of the galvanic coating is typically not as strong as that of PVD adhesion. This double coating system has advantages at lower loads and for corrosion prevention. Cemented carbide can be coated with no problems. But if machined, coolants with cobalt inhibitors should be used to prevent cobalt leaching. Sintered metals with open pores cannot be coated because residues from the sintering process outgas in a vacuum, interfering with the plasma process. Metallized or conductive ceramics can be coated. Plastic is not an option for these functional PVD coatings.

A coating as a part of a mechanical system can boost reliability and service life in critical functions. The low-friction coefficients permit operation with deficient lubrication as well as emergency operation in case of lubricant loss. Coatings also let systems function where lubricants are not permissible as in cryogenic, vacuum, food, medical, clean-room, and vacuum applications. Depending on the situation, coatings might possibly replace such expensive materials as cemented carbide and ceramics. The better performance of the tribo-system can lead to greater load-bearing capacity, lighter designs, and better efficiency. Improved corrosion resistance is another desired aspect in many applications.

MAKE CONTACT
Oerlikon Balzers Coating USA Inc., (716) 564-2788, oerlikon.com/coatingservices/us