Surface and Colloid Chemistry in Advanced Ceramics Processing

Robert J. Pugh, Lennart Bergstrom, Robert J. Pugh, Lennart Bergstrom

• Publisher: CRC Press
• Year: 2017
• Print ISBN:
  9780824790981
• eBook ISBN:
  9781351412667
• Topic: Physical Sciences
• Subtopic: Chemistry
• Index: Chemistry

Emphasizes the importance of surface and colloid chemistry in the manufacture of high-performance ceramics. Examines processing-property relationships, powderproductionandcharacte6zation,the dispersion properties of powders in liquids, the rheology of concentrated suspensions, and the surface and colloid chemistry aspects of the most widely used forming methods.

1

Surface and Colloid Chemistry in Ceramics: An Overview

ELIS CARLSTRÖM Swedish Ceramic Institute, Gothenburg, Sweden

I. High-Strength Ceramics

II. Increasing the Fracture Toughness

A. Particle reinforcement

B. Fiber and whisker reinforcement

C. Processing requirements for fiber and particle reinforcement

III. High-Temperature Properties

A. Processing requirements for high-temperature properties

IV. Sintering of High-Strength Ceramics

A. Sintering behavior

B. Homogeneity in ceramics

V. Forming

A. Pressing

B. Slip casting

C. Injection molding

D. Extrusion

VI. Functional Ceramics

References

By using techniques from colloid chemistry, defects can be removed and the strength of ceramic materials increased considerably. Colloid chemistry techniques can also be used to enhance homogeneity in the mixing of ceramic powders and to improve sinterability, and they play a crucial role in formation of ceramic powder compacts. Synthesis of ceramic powder is yet another area of application of surface chemistry. A number of these possibilities are discussed briefly in this overview and in greater detail in the following chapters.

I. High-Strength Ceramics

Ceramics are brittle materials at moderate temperatures. This brittleness has been a major obstacle in the production of high-strength ceramics, but owing to their superior high-temperature and wear properties, great efforts have been made to produce high-strength ceramics.

A brittle material is not necessarily a low-strength material. Brittleness means that when the fracture stress is exceeded the material fails, with no extra energy required. The failure is instantaneous and is thus often called a catastrophic failure (Fig. 1). A stress and strain diagram can be used to describe this. As tensile stress is applied to the material, the strain increases linearly. If we apply the same stress to a metal, we observe a yield stress. Above the yield stress, the metal deforms plastically and only after a considerable amount of plastic deformation does the fracture occur (Fig. 2).

The strength of a brittle material is controlled by local stress concentrations. If no plastic deformation can occur, a local stress concentration can only be relieved by fracture. If we put a brittle material under tensile stress, we find local stress concentration around cracks or other defects in the material. Any defect in a brittle material will cause a local stress concentration when the material is subjected to stress (Fig. 3).

The strength of a ceramic material can be described by Griffith's equation [1]:

FIG. 1 Stress-strain curve of a brittle material.

Fig. 2 Stress-strain curve of a metallic material.

$$\sigma ={YK}_{{\text{Ic}}^{c^{-1/2}}}\left(1\right)$$

where c is the defect size, K_Ic the fracture toughness, and σ the fracture strength. The Y factor is a constant that depends on the position and shape of the defect.

This provides a simple recipe for a high-strength ceramic. To achieve a strong material, we either have to increase the fracture toughness or decrease the size of the defects. In theory this is very simple; in practice there are a number of interrelated difficulties that have to be solved.

As one single defect can limit the strength, a considerable variation in strength from sample to sample will occur. Because of this, the measure of the strength of a ceramic material has only really a valid mean value when associated with a description of the variation in strength such as the Weibull modulus [2]. Processing of ceramics with colloid chemistry methods aims to increase both strength and reliability. The strength increase comes from reduction of the maximum defect size, while reliability is increased by reducing the statistical variation of defect sizes, i.e., reducing the spread in strength.

FIG. 3 A defect in a brittle material causes a stress concentration when the material is subject to tensile stress.

In order to describe the statistical variation of strength in brittle materials, Weibull used the analogy of a chain in which the weakest link determines the strength. In a ceramic material, the links are represented by defects such as cracks, inclusions, and pores, all of which give rise to stress concentrations and a consequent risk of brittle failure. The strength of a brittle material can be deduced from the fracture toughness and the size (and shape and position) of a critical defect. With a given fracture toughness, an increase in strength and reliability of a brittle material is achieved by decreasing the size and number of defects in the material.

There are some exceptions to this phenomenon. Transformation toughened ceramics and fiber/whisker reinforced ceramic materials can be relatively insensitive to flaw size. Examples of such materials are optimally aged transformation toughened Zr0₂ [3] and fiber composites with very weak fiber-matrix interfacial bond strengths [4].

The defects in a ceramic material are often caused by defects or impurities in the starting powders. Defects are also introduced during processing before firing, during firing, and during machining to final shape. A variation on the chain analogy theme can be used to describe the introduction of defects during fabrication of ceramics. Powder synthesis and preparation, powder processing and forming, sintering, and machining are all links in this chain. Additions of a new link to this chain can never increase the strength of the chain, only maintain it or decrease it. In the same way, each of these processing steps has a very limited if any ability to decrease the number of defects produced by a preceding step. Sintering, for example, can never remove foreign inclusions, but it can introduce new defects such as large grains owing to abnormal grain growth. Powder processing and forming of powder compacts are to a large extent controlled by surface and colloid chemistry mechanisms. This makes application of surface and colloid chemistry techniques among the most important steps in the production of high-strength ceramics.

Kendall, Alford, and Birchall [5] measure the variation in strength (Weibull modulus) of an alumina ceramic before and after sintering. Their experiments show that the strength increases, while the variation in strength (Weibull modulus) of the bodies remains unchanged. This indicates that the same defects limited the strength in the green and in the sintered state in this system. Moreover, the size of these defects remains the same during sintering. Hot isostatic pressing is capable of removing some types of processing flaws such as pores and cracks [6]. Unfortunately, certain other types of defects, such as pores at the surface or just below the surface, cannot be removed [7].

In order to increase the strength and reliability of ceramic materials, the links in the chain must be examined beginning from the starting materials, and efforts must be made to remove or restrict defects or defect formation at each step [8]. In doing this, the process can be simplified somewhat by examining the fracture surfaces of a material to find out what the dominating fracture origins consist of [9,10]. Efforts can then be directed to removing these defects rather than trying to remove every conceivable type of defect that might possibly occur.

Most ceramics are formed as powder compacts and made dense by sintering. Powder used for fabrication of high performance ceramics will only sinter to dense materials if the particle size is small enough. In practice this means particles of sizes 5-0.05 μm. In this size range colloidal forces will often be more important than the force of gravity. For this reason, techniques of surface and colloid chemistry are some of the most important tools in ceramic processing.

By using colloid chemistry techniques, defects can be removed and the strength of the ceramics increased considerably. As described below, there are a number of techniques available. There are also examples in the literature, where colloid approaches have indeed been used to increase the fracture strength of ceramics. Lange et al. [11] have shown that the strength of composites in the alumina-zirconia system can be increased using the colloidal approach. Linde et al. [12] have shown that this can also be done in a nonoxide ceramic system. Alford, Birchall, and Kendall [13] have shown that notable strength improvement in a pure alumina system can be achieved. They report strengths in the range of 1 GPa with a high Weibull modulus preserved. This should be compared with the typical strength of 300-400 MPa for commercial aluminas. These are some of the practical examples that show that both the strength and the reliability of ceramics can be improved using colloid chemistry methods.

II. Increasing the Fracture Toughness

Another way of increasing the strength is to increase the fracture toughness [14]. For a single phase ceramic, the chosen phase and grain size determine the fracture toughness. However, many single phase ceramics have a grain boundary interphase of special composition. By controlling the amount and properties of the grain boundary phase, the fracture toughness can be optimized. This gives a limited space for increase of the fracture toughness, often at the expense of the high-temperature properties.

A. Particle Reinforcement

More drastic ways of increasing fracture toughness include particle and fiber (whisker) reinforcement. Particle reinforcement works by crack deflection [15], microcracking, or transformation toughening [16]. Crack deflection means that cracks are deflected by the second phase particles and have to work their way around these particles, losing some of their propagation energy. A microcrack that intersects with a crack path often stops the crack propagation, as it tends to deflect and split the crack in two. Transformation toughening involves using a phase (second or major) of particles that can transform. When the transformable particle is subjected to the tensile field at the crack tip, it transforms an...

Table of contents

• Cover Page
• Title Page
• Copyright Page
• Contents
• Preface
• 1 Surface and Colloid Chemistry in Ceramics: An Overview
• 2 The Chemical Synthesis of Ceramic Powders
• 3 Surface Chemical Characterization of Ceramic Powders
• 4 Dispersion and Stability of Ceramic Powders in Liquids
• 5 Rheology of Concentrated Suspensions
• 6 Surface Chemistry in Dry Pressing
• 7 Surface and Colloid Chemistry in Ceramic Casting Operations
• 8 Interfacial Aspects of Ceramic Injection Molding
• Index