Zinc-Aluminum Alloys as Tribomaterials

The high-aluminum zinc-based alloys comprise a new family of die-casting alloys that have proven themselves in a wide variety of demanding applications. These alloys feature clean, low temperature and energy-saving melting, excellent castability, high as-cast strength and hardness, corrosion resistance and equivalent or even superior bearing and wear properties as compared to standard bronze bearing.

Many of such applications experience sliding wear situations wherein sliding of the alloy occurs against steel components. Though conventional bearing materials, especially bronzes, are still dominating in industrial practice, in the past two decades there is a growing number of published information about results of investigation on their substitution by zinc-aluminum alloys.

Designation of these Zn-Al alloys by the ASTM B86 is ZA-12 (UNS Z 35631, EN 1774 ZnAl11Cu1) and ZA-27 (UNS Z 35841, EN 1774 ZnAl27Cu2). Chemical compositions are given in Table 1. High strength alloy among family of Zn-Al alloys is reported to have properties equivalent to some aluminum alloy. Sliding wear of zinc-based alloys has been studied extensively, but the role of microstructure via heat treatment on the wear properties attracted attention only in a few papers. Considering these facts, an attempt was made to access the effect of heat treatment on the microstructure, mechanical properties and wear behavior of ZA-27 alloy under lubricated and dry sliding conditions.

Also, tribological characteristics of Zn-Al alloys have been investigated in order to clearly establish the tribological potential of Zn-Al alloys. The tribological parameters (parameters of friction and wear) of the Zn-Al alloys are compared to parameters of the lead-tin bronze (composition in Table 1), as a widely applied conventional bearing material.

The basic motive of such investigations is of course of economic nature. Namely, the Zn-Al alloys are characterized by significantly lower price. Besides that, these alloys can successfully be machined by standard casting procedures, like sand casting, centrifugal, permanent and continual casting. Total savings of substitution bronzes with these alloys are estimated up to the level of 35-90%.

The concept of application of Zn-Al bearings as substitution for the bronze ones is not new. The first experiences are related to the period of the Second World War, when different Zn-Al alloys (with only 30 % of AI) were used instead of bronze, primarily due to lack of copper. Besides for bearings, the Zn-Al alloys were applied also for other machine elements, like the worm gears, components of hydraulic installations, etc.

Special importance in development of Zn-Al alloys during the seventies and eighties has the International Lead and Zinc Research Organization (ILZRO).

The ZA-27 alloy was prepared by the liquid metallurgy route. The melt was overheated to 580°C and poured into a preheated Al₂O₃-based ceramic shell mold prepared for investment casting of tensile test specimens and rectangular specimens for wear tests. The as-cast specimens were annealed for 3 hours at 370°C, followed by water-quenching or furnace-cooling.

The microstructure of the as-cast alloy is distinguished by the presence of cored dendrites. The dendrite structure of water-quenched specimen is rather changed, i.e. dendrite core together with interdendrite regions are reduced to a rather small fraction, whereas a fine mixture of constituents prevails in the microstructure. The furnace-cooled microstructure exhibits a distinct pearlite-like morphology with the complete absence of dendrites.

It should be noted that the water-quenched specimens show higher hardness and strength and a marked increase in elongation compared to as-cast and slowly-cooled specimens (furnace-cooled). On the other side, furnace-cooled specimens exhibit the lowest strength and elongation.

The as-cast specimen exhibits a typical dendrite structure where dark areas represent dendrite cores, a f.c.c. aluminium-rich solid solution (? phase), while grey areas surrounding dendrite cores are composed of a fine scale eutectoid mixture of ? and a h.c.p. zinc-rich (? phase). In addition to ?+? eutectoid, an intermetallic CuZn₄ h.c.p. (? phase) spreading like a continuous dark tiny web is present in interdendrite regions. This obviously inhomogeneous structure is a result of solidification under nonequilibrium conditions, i.e. under high values of temperature and concentration gradients.

These parameters have a strong effect on the size and the shape of dendrite branches, chemical composition of dendrite cores, ramification of dendrite branches etc.

The most interesting result concerning the effect of heat treatment is that specimens quenched from 370°C into the water possess high elongation, maintaining tensile strength slightly above the level of as-cast specimens. Slowly cooled specimens are characterized by low strength and low elongation. Considering the microstructural changes during heating and cooling, the increase in elongation in quenched specimens could be ascribed to the very fine dispersion of microconstituents. In the slowly cooled specimens beside coarse eutectoid lamellae, globular and plate-like ? and ? particles precipitated at grain boundaries may be the origin for the micro cracks formation which leads to the lower elongation.

Lubrication strongly affects the wear behaviour. In comparison to dry sliding, the wear rate under lubricated conditions is much lower indicating a different wear mechanism. In this case the interaction of the lubricant and sliding surfaces is more important factor than hardness and strength of the alloy.

Contrary to dry sliding, slowly cooled specimens exhibit the best wear properties. The soft mixture of a coarse ?+? eutectoid lamellae has lubricating properties mostly due to the presence of ? phase. Apparently, this microstructure contributes to the overall lubrication effect and decreases the material loss. The other two examined microstructures have a lower fraction of ? and ? phases and, consequently, a lower resistance to wear.

One other factor also influences the different behaviour of lubricated and dry-tested specimens. It was reported that the sliding effects produced by different microconstituents are effective as long as the alloy is thermally stable during tests. During dry sliding much higher temperatures were developed and, as a result, the point where the thermal condition of the alloy becomes unstable is reached earlier causing higher wear rate.

Hereby some test results are presented, related to the possibility of substituting bronzes. Samples for the tests included Zn-Al alloys ZA-27 and ZA-12 and lead-tin bronze CuPb15Sn8 (ASTM ? C93900).

Tribometric tests were performed on the computer-supported tribometer. Computer support to experiment was enabled by application of the Burr -Brown PCI 20000 data acquisition system integrated into PC computer and general-purpose LABTECH NOTEBOOK software package.

Based on requirements to realize the contact and relative motion type similarity on the model and the real system, for tribological modelling of sliding bearing was chosen (in tribometric practice the most present) pin-on-disc contact scheme with continuous sliding. As in real tribological system bearing/journal, the tribologically critical contact element is bearing, on the model the stationary pin corresponds to it, which is, due to small degree of covering, tribologically more critical contact element of the contact pair on the model.

For a comparison of the tested alloys the volume wear rate was chosen, since it is directly related to linear wear, which is, in the system of journal bearing, responsible for increase of clearance and loss of the bearing's working ability (Figures 1, 2, 3).

Figure 1: Wear rate of ZA-12 vs. applied pressure

Figure 2: Wear rate of ZA27 vs. applied pressure

Figure 3: Wear rate of Cu Pb15Sn8 bronze vs. applied pressure

It can be found clearly from the graph that the wear rates of the tested alloys increase with the increase in applied load, what was expected in conditions of the boundary lubrication. The established decrease of the friction coefficient with increase of the contact pressure, in tested bearings materials, is in compliance with known principles of the frictional behavior of the metallic materials in the boundary friction conditions.

There, the tendency of the friction coefficient decrease with increase of pressure is significantly more prominent for the ZA-27and CuPb15Sn8 bronze. For all the three levels of contact pressure ZA-27 has the lowest friction coefficients.

Due to possibility of direct comparison of tribological properties of materials, which are obtained in different contact conditions, it is usual to use the parameter which represents the wear per the unit of the normal contact load and the sliding distance. This parameter is known as the specific wear rate or wear intensity. Values of this parameter for the compared alloys are given in Table 2, where is for comparison, also for this case, used the volume wear.

Established levels of tribological characteristics, from the aspect of wear resistance, as well as friction coefficient, confirm that both commercial Zn-AI alloys (ZA-12 and ZA-27) represent respectable tribological materials for conditions of boundary lubrication that are characteristic for high loads and low sliding speeds.

Obtained results show that chemical composition varying from ZA-12 to ZA-27 provides very significant improvement of friction and wear characteristics. In accordance with that, Zn-Al alloys can be used in broad range of contact conditions.

The main question is related to the possibility of substituting traditional bronze. Obtained results point that, at the same testing conditions, for all applied loads, lower wear rate, for the given friction distance, corresponds to material ZA-27 with respect to material CuPb15Sn8. It can be seen that the larger differences in the wear rates correspond to lower contact loads.

Based upon above described tribological performances testing Zn-AI alloys have been tested and applied in a variety of engineering applications. These alloys have been found to be cost effective tribomaterials, especially as substitutes of bronzes for bearing purposes.

Good carrying capacity and wear resistance enabled application of these alloys for maintenance of various tribomechanical systems in power plants, like the sliding radial and journal bearings, various bushings, nuts for the screwed spindles, guides, etc. Zn-Al alloys bearings have shown particularly good exploitation characteristics in electro-locomotive motors and mining tribomechanical systems, functioning in conditions of high loads, relatively small speeds, and boundary lubrication.