Friction and Wear in Space Vacuum - A Review

Introduction

The tribology of contacting metallic surfaces is greatly influenced by the presence or absence of an oxide film. Hence, it is this aspect that marks the essential difference between the wear and friction of materials operating in vacuum as opposed to air. In addition, the presence or absence of water vapour can radically alter the frictional behaviour of some materials, as can extreme temperatures.

Wear

There are several types of wear:

• Abrasive wear; common in industry, but not relevant to Space applications
• Adhesive wear; characteristic of metal-to-metal contact (or polymers/ceramics etc) where the normally protective oxide is a crucial factor
• Fretting; caused by localised contact and adhesive/abrasive wear
• Scuffing; a severe adhesive wear characteristic of high speed lubricated contacts like cams and tappets; not relevant to Space applications
• Corrosive wear; a combination of corrosion and wear which is unlikely in vacuum

Adhesive Wear

Adhesive wear between metallic surfaces is characterised by two distinct forms:

• Mild Wear, where sliding conditions are such that the natural oxide can continuously reform as it is worn and provide a degree of lubrication.
• Severe Wear, where conditions are such (high loads and speeds) that the oxide is penetrated and gross metallic bonding occurs. Typically, wear rates can be a factor of 1,000 higher than in the mild wear regime and, for soft materials with thin, brittle oxides (stainless steel, aluminium and titanium), the wear can proceed to s seizure situation referred to as galling

Severe wear can be combated by hardening the surface, giving support for the natural oxide and limiting the growth of the surface contact area under deformation and shear. Hence, any through hardening, case hardening, nitriding or hard plating process will reduce the wear.

In Space, however, any surface oxide will not replenish once disrupted, and there is a significant risk of severe wear and seizure between parts that are not hardened or lubricated, even under low load, slow running situations.

With polymers like PTFE, polyacetal or nylon, wear in vacuum is similar to that in air, with the mechanism depending mostly on the transfer of polymer to the counterface. The presence or absence of an oxide on that counterface has marginal importance. Likewise, hard cermets like tungsten carbide or ceramics like chromium oxide will show little change in wear rate in vacuum. However, their high hardness may result in abrasion of an oxide-free metallic counterface.

Fretting

Fretting is a complex sequence of localised adhesive wear caused by small (100µ or less) vibration movements or impacts, the production of wear debris trapped within the small contact area, the oxidation of that debris, and the subsequent abrasion by that oxide. It is common for the situation to accelerate, with the abrasion causing yet more debris.

In Space vacuum, the oxidation step is absent and the fretting problem may be reduced to one of adhesive wear, with the debris not contributing significantly to subsequent wear. however, the real danger in vacuum is of a seizure situation, so that the small sliding motions can no longer be accommodated and unacceptable stresses may lead to fatigue or other mechanical problems. Parts that might fret on board a Space device must be effectively lubricated.

Friction

The friction coefficient between two sliding materials is defined as:

µ = ^F/_L

Where L is the applied load and F is the resulting frictional force.

Friction in air

In a terrestrial environment, the dry sliding friction between most metallic parts is greatly influenced by the natural surface oxide which, for most parts, acts as a partial lubricant and maintains a coefficient of between 0.2 and 0.6. Some materials like gold, which form little or no oxide, give much higher friction and light weight materials like aluminium and titanium, where the oxide is very thin and brittle, are prone to sudden galling as the protective film is disrupted during the wear process.

Friction in vacuum

In vacuum, the natural protective oxide is absent and, without lubrication of surface coatings, many sliding metallic parts would exhibit extremely high friction coefficients, perhaps up to 5 or higher. Even with hard protective coatings or treatments (such as nitriding, hard chrome, anodising, etc) dry friction will be much higher than in air. Hence, either a low vapour pressure liquid lubricant or a low friction solid lubricant is essential.

Solid lubricants

The two main solid lubricants for Space vacuum use are PTFE and Molybdenum Disulphide and both provide a low friction coefficient. In fact Molybdenum Disulphide, in the absence of the detrimental effects of water vapour, can provide some of the lowest dry friction coefficients ever measured; 0.015 to 0.02. PTFE gives a friction value in the range 0.05 to 0.10 and is essentially independent of air or vacuum. However, at -70'C there is a structural transition which increases the friction to 0.2 or more, so that operation of components in eclipse conditions may be a problem.

One material commonly used to provide low friction in air is graphite. However, this low friction depends on the presence of moisture or oxygen and, in vacuum, the friction coefficient may be a high as 0.8.

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