Thermal Shock of Ceramics Author: John Cotton
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Thermal shock is defined as cracking in a component subject to rapid changes changes in temperature.
What is Thermal Shock and Why is it Important? Pouring boiling water into a lead crystal glass is likely to cause it to shatter. Similarly using the wrong type of dinnerware in an oven can cause it to crack. Opening a furnace before it has cooled down can crack the fired components or even the furnace lining. These are all examples of thermal shock. Thermal shock damage occurs when components are subjected to rapid changes in temperature. temperature. This leads to thermal gradients and differential expansions within different regions of the component.
Importance of Thermal Shock Resistance (TSR) in Ceramics Ceramics have high melting points and hence they are often used in high temperature temperature situations. They are often subjected to rapid temperature changes. Their thermal expansion expansion coefficient is low but they are stiff and brittle and unable to accommodate accommodate high strain which result from different amounts of thermal expansion in different parts of the component. component. Even without rapid change in temperature ceramics and other brittle materials can suffer damage if they are subjected to large temperature gradients.
What Happens to a Material in a Temperature Gradient?
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Thermal Gradients If the temperature of a component remains uniform throughout then no distortion occurs. If external temperature changes lead to thermal gradients, then the effects of expansion expansion or contraction cause distortion. Since ceramics are relatively brittle, distortion can lead to cracking as the failure strain is exceeded. exceeded.
Properties Leading to Good Thermal Shock Resistance Low expansion coefficient coefficient to reduce the stress associated associated with a temperature gradient •
High thermal conductivity to conduct heat away and minimise temperature gradients •
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High toughness or work of fracture to improve resistance to crack propagation propagation
High strain to failure to accommodate accommodate thermal stress and prevent catastrophic failure •
Low elastic modulus to minimise the stress associated associated with differential differential expansion •
Conditions for Good Thermal Shock Resistance •
Linear thermal expansion characteristic - i.e. absence of phase changes changes
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Small component size
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Uniform heating - i.e. no external temperature gradients gradients
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Slow heating rate
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Freedom from external loading
Thermal Expansion of Refractory Ceramics
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Quantifying Thermal Shock Resistance How do we decide which materials have good t hermal shock resistance and which are not so good? •
TSR is a performance performance measure influenced by a number of material properties
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The ‘values’ calculated depend on the type of test employed
The shape and dimensions of the component component significantly influence the calculation •
Quantifying TSR – The Hasselman Parameters In an attempt to produce a quantitative ranking of materials with respect to their TSR, the Hasselman parameters have been defined. High values of R indicate good TSR performance. If the surface temperature of a body is rapidly changed from T 0 to T1, the stress generated at the surface in an infinitely thin layer is
The basic thermal shock parameter R is the maximum temperature change which can be withstood without the stress generated generated exceeding the fracture stress.
Modifications to R - R’ & R” •
R does not reflect reality however we define R’ and R’’
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R’ reflects behaviour behaviour in a constant heat flux and incorporates the effect of thermal conductivity •
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R’’ also takes into account the effect of density and specific heat
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R -R” Limitations The Hasselman parameters R, R’ and R’’ only give an approximate indication of relative performance performance •
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They assume simple conditions and hence do not reflect in service situations
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The calculations are based on relatively simple shapes
These three thermal shock parameters define the point at which the fracture process is initiated •
Hasselman Parameters R’’’’ & Rst the Energy Balance Approach In an attempt to reflect realistic material behaviour and to encompass different types of material a fracture mechanics approach can be taken •
This approach is concerned with the propagation of pre-existing flaws under stresses generated during thermal shock •
It distinguishes between dynamic propagation of microcracks and quasi-static propagation of large cracks •
In thermal shock, elastic energy is stored in the material material and released released during fracture. This energy can be related to the work done in generating generating new fracture surface area (this may be many cracks or a single crack) Hence we can relate work of fracture to TSR by another parameter R’’’’
Hasselman Parameter Rst For materials such as coarse grained refractories, where quasi-static crack propagation propagation occurs, the thermal stress crack stability parameter applies.
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Hasselman Parameters - Summary
Symbols σ = fracture strength Ε = Young’s modulus ν = Poisson’s ratio α = thermal expansion coefficient λ = thermal conductivity ρ = density γ = fracture energy Wf = work of fracture R = Hasselman TSR parameters
Thermal Shock Resistance of Materials Material
Properties Giving Good TSR
Fused Silica
Very low thermal expansion
Silicon Carbide
High thermal conductivity conductivity Low expansion coefficient coefficient
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High thermal conductivity conductivity Low expansion coefficient coefficient High work of fracture High strain to failure
Silicon Nitride
High thermal conductivity conductivity Low expansion coefficient coefficient High toughness
Alumina
High thermal thermal expansion Low toughness toughness
Zirconia
Low thermal conductivity conductivity High thermal expansion
Glasses
Low toughness Low thermal conductivity Low strength
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About About Cera Ceram m Ceram is an independent expert in innovation, sustainability and quality assurance of materials. With a long history in the ceramics industry, Ceram has diversified diversified into other materials and other markets including aerospace aerospace and defence, medical and healthcare, minerals, electronics and energy and environment. Partnership Partnership is central to how we do business; we work with our clients to understand their needs so that we can help them overcome materials materials challenges, develop new products, processes and technologies and gain real, tangible results. Headquartered Headquartered in Staffordshire, Staffordshire, UK, Ceram has approved laboratories laboratories around the world.
About About the the Autho Author r John Cotton Expertise in: Aerospace & Defen Defence; ce; Ceram Ceramics; ics; Advanced Materials Advanced Advanc ed Materials Consultant John is a Chartered Engineer who holds a Degree in Applied Physics and is a Fellow of the Institute of Mining Minerals and Materials (IOM3). John serves on the Ceramic Science Committee of IOM3 and is a member of Peer Review College for the Engineering Engineering and Physical Sciences Research Council (EPSRC). John also acts as a Technology Technology Translator Translator for Materials KTN. With over thirty-five years of experience experience in advanced advanced materials – specialising specialising in refractories refractories and technical ceramics ceramics at Ceram, John is an expert in all aspects of materials R&D and problem-solving. From identifying and solving production issues to advising on application design and performance, John has worked with manufacturers, manufacturers, systems integrators and end-users to make a real difference to their businesses. John has contributed to several materials textbooks, composed a large number of papers and is a frequent presenter at conferences worldwide. Advanced Advanced Materials Throughout his term at Ceram John has worked with a range of advanced materials including including both monolithic and composites for applications applications such as fuel cells, lightweight lightweight materials for airframe and sporting goods, as well as sensors, actuators, and high temperature and wear resistant components. Aerospace and Defence John's experience in aerospace and defence materials incorporates ceramic armour, lightweight lightweight and high temperature composites and coatings for thermal and corrosion management. Ceramics John has been involved in a range of ceramic projects including the development of sinterable sinterable silicon nitride ceramics, evaluation of ceramic materials as electrochemical electrochemical gas sensors, design and manufacture of ceramics for diesel engine components, components, and design of dies and development of extrusion technology for the production production of thin ceramic and metal powder tapes.
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