EE Klaus Professor Emeritus of Chemical Engineering and Fenske Faculty Fellow

Perm State University

Abstract

Advances in the improved efficiency of internal combustion engines is currently restricted by the materials available for construction and lubrication of these devices. The development of wear and heat resistant materials for bearing surfaces represents a partial solution to this problem. Wear coefficients of these ceramic materials are still several orders of magnitude above the values required for current diesel engine life. Lubricants that can be utilized over the range of 300 to 1200°C are needed to complete the solution to this problem. In addition to good wear resistance, low friction is required to retain current engine efficiency.

It appears that a continuous supply of lubricant will be required to supplement ceramics, hard coats, solid-film lubricants, and combinations of these to satisfy the requirements of low heat rejection engines. Reactions in boundary lubrication take place at greater than 300°C to produce "friction polymer" which in turn provides an easily sheared film. Exploration of this chemical reaction approach appears to have some promise in the development of high temperature lubrication. Thermal stability limits of the C-C bond (371°C) and the aromatic ring (450°C) places limits on the conventional liquid delivered recirculating lubrication system. In almost all cases, oxidative stability is less than the thermal stability of these lubricants. These facts only suggest that long life recirculating liquid lubrication systems have a low probability of success. The high temperature reactions {both thermal and oxidative) over the range of 300 to 1200°C

indicate that an easily sheared lubricant film may be provided by many of the conventional lubricants.

In order for the rapid chemical reactions that occur in the 300 to 1200°C temperature range to provide lubrication without forming excessive deposits or destroying the bearing surface, both the rate of delivery and the resultant chemical reactions must be controlled and understood.

Lubrication based on these considerations could be applied with the fuel, the air or directly to the piston ring-cylinder zone. Lubrication by these methods would require bearing designs compatible with boundary lubrication.

Introduction

Liquid lubricants for use at temperatures up to 250°C are available for a wide variety 'of applications and provide a high degree of reliability. Lubricants (liquid and solid) for applications above 250°C for the long life required by the transportation industry are not generally available. The lack of a reapplication mechanism for the solid lubricant severely limits its use in long term applications involving substantial loading (1). Liquid lubricants for this temperature range generally produce excessive degradation products by continued exposure to air and high temperatures. Since good boundary lubrication is provided by a wide range of liquid lubricants, it is evident that many of these liquid lubricants can and do work well for short time exposure to temperatures of at least 350°C.

This temperature has been shown by chemical kinetics to be the minimum temperature achieved in effective boundary lubrication

(2). In the case of less effective boundary lubricants, higher temperatures than 350°C are achieved. It is also clear that significant chemical reactivity does occur at boundary lubrication temperatures

(3). These data suggest that liquid lubricants can be used for temperatures above 250°C if methods of lubricant application are designed so that only enough lubricant is supplied to the bearing system to provide for lubrication. It is primarily the excess lubricant and the recycling of degraded lubricant that results in excessive deposits in the bearing system. When lubricants delivered as sprays or mists to the bearing still produce excessive deposits, a controlled system to deliver only the amount needed for lubrication is required to solve the excess deposit problem. Delivery of such small but precise amounts of lubricants can be achieved by vaporizing the lubricant of interest in an inert carrier gas (4). Using this methodology, precise amounts of lubricant can be delivered to a bearing system at temperatures in the range of 250 to 1600°C from 0 mole percent to the saturation vapor pressure of the lubricant in the carrier gas at the temperature of the carrier gas. This system has been called vapor phase or vapor-delivered lubrication (5,6,7). This lubricant delivery system and its limitations will be discussed in this paper along with the use of liquid and solid lubricants for systems involving metal, ceramic, ceromet, hard coat, and combinations of these bearing types.

Applications

Surveys have shown that better high temperature lubricants are needed for more energy efficient metal forming processes and low heat rejection (adiabatic) diesel engines (8). In the case of metal forming operations including near net shape processes, lubricants with operating limits in the range of 200 to 1600°C are needed. In the low heat rejection diesel engine, temperatures in the range of 250 to > 600°C are of interest. Primary emphasis will be given to engine lubrication in this paper, but the discussions apply equally well to metal-forming lubrication. Lubrication systems suggested for these high temperatures include: 1)

liquid, 2) solid film, 3) composites, 4) hard coats, 5) ceramics, and 6) ceromets. All systems except liquids have been proposed to be run without a continuous replacement method. Liquids suffer from excessive degradation when run in the conventional recirculation mechanism but show promise when delivered continuously at appropriately low rates.

The wear coefficients required in state-of-the-art automotive and diesel engines are shown on Table 1 (9). At the bearing loadings involved in engines, lubrication by systems 2 through 6 fall several orders of magnitude short of the performance required in automotive and

Table 1

Current State of the Art Platon Ring Wear Coefflolenta

Gasoline 0.28 to 2.57x1010him3/N

Diesel 0.30 to 1.32x10'W/N

diesel engines (10). In the case of liquid lubricants, conventional recirculating systems produce excessive deposits for low heat rejection engines. In fact, current liquid lubricants are already borderline for heavy-duty diesel engines (11). As a result, a new concept of lubricant delivery limiting the amount to the minimum needed to supply the easily sheared film has been suggested as a stand-alone system or as a system to be used in conjunction with systems 2 through 6. The goal of this system is to supply the liquid lubricant as a vapor in a carrier gas at or near the minimum rate required to produce by condensation and/or chemical interaction a lubricant film at the bearing surface. The amount of lubricant resulting from this delivery system should be enough to provide low friction and wear without a build up of degraded lubricant in the bearing area. Lubrication by this type of system requires a better understanding of the behavior of liquid lubricants at elevated temperatures. This approach to high-temperature lubrication involves chemical reactions among the lubricant, environment, and bearing surface.

Liquid Lubricant Oxidative Stability

Automotive, Diesel and gas turbine lubricants generally fail by an oxidativie process which results in oxidation products which produce viscosity increase and deposit formation. The Penn State Microoxidation Test has been developed to simulate the oxidative behavior of the lubricant in contact with the lubricated bearing surface (12). The simple glass test equipment is shown in Figure 1. The standard test procedure involves a small sample of lubricant (usually 40 pi) placed in a thin film on the catalyst test cup which is generally made from the desired bearing material. The test system is then exposed to the desired atmosphere (usually air), and the desired test temperature for a given time. After the test is completed and the catalyst containing the lubricant is cooled to room temperature, the remaining liquid lubricant is dissolved in a solvent such as tetrahydrofuran and analyzed quantitatively by gel permeation chromatography. The catalyst is weighed to determine the amount of insoluble sludge and varnish formed. The analysis of the liquid product can include spectrographs analysis by IR, NMR, and atomic absorption spectroscopy. By selecting test conditions appropriate for the desired commercial application, excellent correlations have been developed between the Penn State Microoxidation Test and the 3C or 3D SAE engine sequence test (13); heavy-duty Caterpillar diesel engine performance (11); aircraft gas turbine performance; peak power-generating equipment; electric contact lubricants; and magnetic storage tape lubricants (14,15). This test system has also been used to study the kinetics of the oxidation reactions in an attempt to better understand the relative effects of temperature (16) and the catalytic effects of the bearing surface material (17,18,19).

Liquid lubricants in general use that have been considered in this paper are shown on Table 2. The predominant molecular structures in all but the poly perfluoro ethers are based on C-C bonding where the carbon is also bonded to one or more hydrogens. The microoxidation studies have shown a common oxidation path for these materials. This path is illustrated i-n Figure 2. The primary oxidation product appears to be an

PENN STATE MICROREACTOR (ATMOSPHERIC PRESSURE) Figure 1

Table 2

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