Ceramic Matrix Composites
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Questions and Answers

What are the advantages of ceramic matrix composites in terms of wear and corrosion resistance?

Excellent wear and corrosion resistance in a wide range of environments and temperatures.

What is the main disadvantage of monolithic ceramics that ceramic matrix composites are designed to improve?

Brittleness

Why are ceramic matrix composites more expensive to process than other materials?

High processing temperature results in complexity in manufacturing and hence expensive processing.

What is the primary reason for thermal stresses in ceramic matrix composites during cooling?

<p>Difference in the coefficients of thermal expansion between the matrix and the reinforcement.</p> Signup and view all the answers

What is the primary mechanism of crack arrest in ceramic matrix composites?

<p>Crack impeding, which is due to the higher fracture toughness of fibers compared to the matrix.</p> Signup and view all the answers

What is the benefit of ceramic matrix composites in terms of strength retention at elevated temperatures?

<p>Higher strength retention at elevated temperatures.</p> Signup and view all the answers

What happens when a crack propagates in the matrix of a composite material?

<p>The crack will not be able to tear open the fibers and propagate through the fibers.</p> Signup and view all the answers

What is the result of high transverse fracture toughness in fibers during fiber pull-out?

<p>Failure occurs along the fiber/matrix interface due to pulling out of fibers from the matrix.</p> Signup and view all the answers

What is the purpose of Chemical Vapour Infiltration (CVI) in developing Ceramic Matrix Composites?

<p>To deposit solid materials like carbon, silicon carbide, and boron nitride in a porous structure.</p> Signup and view all the answers

What is the difference between Chemical Vapor Deposition (CVD) and Chemical Vapor Infiltration (CVI)?

<p>CVD implies deposition onto a surface, whereas CVI implies deposition within a body.</p> Signup and view all the answers

What is the role of the heating chamber in a Chemical Vapor Infiltration reactor?

<p>It is one of the three main parts of the reactor, along with the feed system and effluent system.</p> Signup and view all the answers

What is the advantage of the Temperature Gradient (TG-CVI) process over the isothermal/isobaric CVI process?

<p>The temperature difference enhances the gas diffusivity, making it a faster process.</p> Signup and view all the answers

Study Notes

Ceramic Matrix Composites (CMCs)

  • Exhibit excellent wear and corrosion resistance in various environments and temperatures
  • Offer higher strength-to-weight ratio
  • Retain higher strength at elevated temperatures
  • Possess higher chemical stability
  • Fail in a non-catastrophic manner
  • Have high hardness and are lightweight

Disadvantages of CMCs

  • Processing routes involve high temperatures, limiting their use with high-temperature reinforcements
  • Designed to improve toughness of monolithic ceramics, which are brittle
  • High processing temperature leads to complex and expensive manufacturing
  • Coefficient of thermal expansion differences between matrix and reinforcement cause thermal stresses during cooling

Toughening Mechanisms in CMCs

  • Crack impeding: crack arresting mechanism due to fibers' higher fracture toughness than the matrix
  • Fiber (or whisker) pull-out: fibers have high transverse fracture toughness, causing failure along the fiber/matrix interface
  • Crack deflection: weak fiber/matrix interfaces deflect the crack

Chemical Vapour Infiltration (CVI)

  • Widely used to develop CMCs
  • Deposits solid materials like carbon, silicon carbide, and boron nitride in a porous structure by vapor decomposition
  • Similar to Chemical Vapor Deposition (CVD), but involves deposition within a body

CVI Process

  • Prepares a ceramic continuous fiber structure (porous preform) as the reinforcement phase
  • Reactant gases or vapors are supplied to the reactor, flowing around and diffusing into the preform
  • Decomposition of reactants fills the space between fibers, forming a composite material

CVI Reactor

  • Consists of three main parts: feed system, heating chamber, and effluent system
  • Can be modified to include forced gas flow and a temperature gradient to avoid restriction of gas flow

Types of CVI Processes

  • Isothermal/isobaric CVI process: uniform temperature and pressure, but slow due to low diffusion rate
  • Temperature gradient (TG-CVI) process: enhances gas diffusivity by leveraging temperature differences

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Explore the advantages and disadvantages of ceramic matrix composites, including their excellent wear and corrosion resistance, high strength to weight ratio, and more. Learn about the properties and applications of CMCs.

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