Erosive Wear of Alumina and Nitride Bonded Silicon Carbide Ceramics

Background

Slurries are transported and processed by a wide range of equipment in the mining, power generation and dredging industries. Centrifugal pumps and cyclones are used extensively in these applications. A major consideration of equipment operators in these industries is the wear life of equipment.

The predominant type of wear in slurry handling equipment is erosion. Studies into the factors contributing to erosive wear have focussed on particle size, shape, impingement angle, impact velocities and material characteristics. Whilst extensive general research has been carried out with the aim of better understanding the erosive process, it has not always been representative of the plant operating conditions used in industry.

Types of Wear

Wear of slurry equipment can be categorised into two major types, these being erosion and abrasion. Erosive wear is the dominant process and can be defined as the removal of material from a solid surface due to the mechanical interaction between the surface and impinging particles in a liquid stream. Abrasion, is the loss of material which occurs when particles are forced against and move along the solid surface. The main difference between the two types of wear is that erosion involves the transfer of kinetic energy to the surface, whilst abrasion does not. This means that in erosion, material removal is a function of the particle velocity squared (or some higher power). Abrasive wear, on the other hand, is a linear function of the particle velocity (and the applied force).

Four different types of wear have been identified by Shook and Roco. These are listed below;

        Directional impingement

        Random impingement

        Sliding bed

        Grinding

The first two types are erosive processes while the third type is more abrasion than erosion. The fourth wear type listed is an abrasion process. Erosive wear depends on the predominant angle of particle impingement with the material surface. Particle impingement angles will vary between 0 and 90 degrees, and depend on both fluid-particle and particle-particle interactions. Directional impingement involves the particles impacting the material surface at a common angle. Examples of this type of wear can be found on impellers and volute liners in slurry pumps, knife gates in gate valves and angled pipe bends.

Wear in Slurry Pumps

The various components in slurry pumps and hydrocyclones can undergo each or all of the wear mechanisms listed previously. Table 1 outlines the different types of wear for various component parts in typical slurry handling equipment.

Table 1. Dominant wear types for slurry handling equipment.

Product Part

Direct Impingement

Random Impingement

Sliding Bed

Grinding

Slurry Pump

Impeller

Volute casing

Side liners

Shaft sleeve

X

X

X

X

X

X

X

X

X

Cyclone

Feed chamber

Vortex finder

Spigot

X

X

X

X

X

X

In establishing the goal of lowering the ownership costs for slurry handling equipment end users, Warman International is committed to a research program aimed at achieving a fundamental understanding of the variables that effect erosive wear, and in particular, under simulated plant operating conditions. Part of this program includes the laboratory and field testing of engineered ceramic materials. This paper outlines an investigation into the erosive wear characteristics and mechanisms of two types of engineering ceramics, namely a 96% sintered alumina and a nitride bonded silicon carbide. Also included are some examples of the successful application of both these materials in an industry application.

Variables

A research project looking at the wear characteristics of two engineering ceramics:

        96% sintered alumina (Taylor Ceramic Engineering)

        Nitride bonded silicon carbide – Refrax (Carborundum Resistant Materials)

And a range of slurries containing different materials such as fused alumina and river sand of different particle sizes.

Particle Size and Impingement Angle

The study has reinforced the “rule of thumb” notion that ceramics should be applied in applications involving predominantly low energy particle impacts. These conditions are achieved under fine, rounded particles impacting at low impingement angles. As particle size and impingement angle increase, so to does the wear rate of thee two engineering ceramics.

Particle Morphology

It was also found that as the impinging particle morphologies changes from round to sharp, the wear rate increased. This has been attributed to higher contact point stresses for the sharper particles.

Wear Behaviour of Alumina

The alumina ceramic investigated consisted of equiaxed, microcrystalline grains of Al2O3. SEM investigations indcate that brittle fracture generally occurs on a macro scale as a result of deformation and slip  occurring within the alumina grains themselves.

Wear of Nitride Bonded Silicon Carbide

The nitride bonded silicon carbide material consist of silicon carbide particles in a silicon nitride matrix. The matrix is softer them the silicon carbide and wears preferentially. The finer the particle, the greater the likelihood of these particles getting in between the hard carbide and matrix phases and wearing away the matrix. This is part of the reason why the wear tests suggested that alumina has better wear resistance then the Refrax material under fine particle impingement conditions.

As the matrix is removed, the carbide grains are left exposed and prone to impact and brittle fracture due to particle impingement. Alternatively, if sufficient matrix is removed the carbide grains can pull out from the matrix after impact from the particles. If the impinging particles are too coarse, cracking on a macro scale similar to that of alumina results.

Summary

Wear rates of engineering ceramics have been found to increase when:

        Impingement angles increase

        The particle size of the impinging materials increases

        The impinging materials increase in sharpness

Furthermore, the dominant mode of wear for ceramic materials is the formation, propagation and intersection of cracks caused by brittle fracture. In the case of nitride bonded silicon carbide, brittle fracture of the matrix can leave silicon carbide grains exposed to fracture or pullout from the matrix.

Note: References are available by referring top the original text (see below).

Primary author: G.C. Bodkin

Source: Abstracted from International Ceramic Monographs, Vol. 1, no. 2, pp. 1397-1403, 1994.

For more information on this source please visit The Australasian Ceramic Society.

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