Microstructural Evolution and Oxidation Behaviour of Spark Plasma Sintered Mn+1AXn Ceramics
Author(s)
Cui, Bai
Type
Thesis
Abstract
MAX phases are promising candidates for high-temperature wear, hypersonic and nuclear
applications. Understanding the formation of MAX phase microstructures is crucial to their
application because they often coexist with phases such as M-A intermetallics, M-X or A-X
binary compounds which are intermediate products of the synthesis. The relation between
processing and fired microstructures in Ti2AlN ceramics made by spark plasma sintering
(SPS) was examined using a range of advanced microscopy techniques. When sintered at
1300 ºC, nearly single-phase Ti2AlN ceramics with elongated (~22×6×6 μm) grains were
obtained. After sintering at 1200 ºC and chemical etching, Ti2AlN nanowhiskers (150-200
nm dia., 1-5 μm long) were exposed in pores coexisting with TiAl, TiN and Ti2AlN grains.
The nanowhiskers are believed to form by diffusion of TiN into TiAl during SPS and are
exposed during the chemical etch. Microstructural studies of Ti3AlC2/W composites prepared
by SPS at 1300 ºC revealed “core-shell” microstructures in which a TixW1-x “shell”
surrounded a W “core” in a Ti3AlC2 matrix. Above 1200 ºC, W reacted with Ti out-diffused
from Ti3AlC2 to form TixW1-x solid solution with crystal structure isotypic with bcc W and
non-stoichiometric Ti3-xAlC2.
The oxidation mechanism of MAX phases is not fully understood which may limit their
applications at high temperature in oxidising atmosphere. To understand the fundamental
nature of the oxidation mechanism of MAX phases requires investigation of the
microstructural evolution of the oxide scale during oxidation. Microstructural development
during high-temperature oxidation of Ti2AlC below 1300 ºC involves gradual formation of an
outer discontinuous TiO2 layer and an inner dense and continuous α-Al2O3 layer. At 1400 ºC,
a mixed outer layer of TiO2 and Al2TiO5 and a cracked α-Al2O3 inner layer formed. During
high-temperature oxidation of dense Ti2AlN ceramics below 1200 ºC layered microstructures
containing anatase, rutile and α-Al2O3 formed on the surface. Above 1200 ºC, more complex
layered microstructures containing Al2TiO5, rutile, α-Al2O3 and continuous void layers
formed. The planar defects formed after Ti2AlC oxidation at 1200 ºC and Ti2AlN oxidation at
1100 ºC for 1h were identified as twins and stacking faults. After heating both Ti2AlC and
Ti2AlN to 1400 ºC for 1h and cooling to room temperature, cracks propagate in TiO2 grains.
Planar defects and cracks may arise from stress generation in the oxide scale. The thermal
stresses formed during cooling may result from thermal expansion mismatch of phases (TiO2,
Al2O3 and Al2TiO5) in the oxide scale, the high anisotropy of thermal expansion in Al2TiO5
and thermal expansion mismatch between the oxide scale and Ti2AlC or Ti2AlN substrate.
Growth stresses formed during isothermal oxidation treatment may arise from the volume
changes associated with oxidation reactions of Ti2AlC or Ti2AlN.
An oxidation mechanism for Ti2AlC is proposed, in which the growth of oxide scale is
caused by inward diffusion of O2- and outward diffusion of Al3+ and Ti4+. The weakly bound
Al leaves the Al atom plane in the layered structure of Ti2AlC, and diffuses outward to form a
protective inner α-Al2O3 layer between 1100 and 1300 ºC. However, the α-Al2O3 layer
becomes cracked at 1400 ºC, providing channels for rapid ingress of oxygen to the body,
leading to heavy oxidation. An oxidation mechanism for Ti2AlN is proposed, which involves
initial reaction with atmospheric oxygen to form oxide phases, demixing of the mixed oxide
phases, and void formation due to the Kirkendall effect and gaseous NOx release. The
oxidation resistance of Ti2AlC (up to 1400 ºC) is better than that of Ti2AlN (up to 1200 ºC).
Stress generation and gas formation appear to play important roles in the different oxidation
mechanisms of Ti2AlC and Ti2AlN.
Microstructural development during high-temperature oxidation of Ti3AlC2/W composites
involves α-Al2O3 and rutile formation ≥1000 ºC and Al2TiO5 formation at ~1300 ºC while
tungsten oxides may have volatilised above 800 ºC. Likely due to exaggerated, secondary
grain growth of TiO2-doped alumina, fine (<1 μm) Al2O3 grains formed dense, anisomorphic
laths on Ti3AlC2/5wt%W surfaces ≥1200 ºC and coarsened to large (>5 μm), dense, TiO2-
doped Al2O3 clusters on Ti3AlC2/10wt%W surfaces ≥1400 ºC, which were more protective
than well-dispersed Al2O3 grains. W may affect the oxidation behaviour of Ti3AlC2/W
composites in two ways: a) beneficially by weakening the Ti-Al bond in Ti3AlC2 by
attracting Ti to form Ti1-xWx resulting in a higher diffusivity of Al which diffuses outward to
form a protective α-Al2O3 layer during high-temperature oxidation; and b) detrimentally by
releasing volatile tungsten oxides so generating pores in the oxide scale. However, at high
temperature (≥1400 ºC) the former beneficial effects appear to be dominant over the latter
detrimental effect.
applications. Understanding the formation of MAX phase microstructures is crucial to their
application because they often coexist with phases such as M-A intermetallics, M-X or A-X
binary compounds which are intermediate products of the synthesis. The relation between
processing and fired microstructures in Ti2AlN ceramics made by spark plasma sintering
(SPS) was examined using a range of advanced microscopy techniques. When sintered at
1300 ºC, nearly single-phase Ti2AlN ceramics with elongated (~22×6×6 μm) grains were
obtained. After sintering at 1200 ºC and chemical etching, Ti2AlN nanowhiskers (150-200
nm dia., 1-5 μm long) were exposed in pores coexisting with TiAl, TiN and Ti2AlN grains.
The nanowhiskers are believed to form by diffusion of TiN into TiAl during SPS and are
exposed during the chemical etch. Microstructural studies of Ti3AlC2/W composites prepared
by SPS at 1300 ºC revealed “core-shell” microstructures in which a TixW1-x “shell”
surrounded a W “core” in a Ti3AlC2 matrix. Above 1200 ºC, W reacted with Ti out-diffused
from Ti3AlC2 to form TixW1-x solid solution with crystal structure isotypic with bcc W and
non-stoichiometric Ti3-xAlC2.
The oxidation mechanism of MAX phases is not fully understood which may limit their
applications at high temperature in oxidising atmosphere. To understand the fundamental
nature of the oxidation mechanism of MAX phases requires investigation of the
microstructural evolution of the oxide scale during oxidation. Microstructural development
during high-temperature oxidation of Ti2AlC below 1300 ºC involves gradual formation of an
outer discontinuous TiO2 layer and an inner dense and continuous α-Al2O3 layer. At 1400 ºC,
a mixed outer layer of TiO2 and Al2TiO5 and a cracked α-Al2O3 inner layer formed. During
high-temperature oxidation of dense Ti2AlN ceramics below 1200 ºC layered microstructures
containing anatase, rutile and α-Al2O3 formed on the surface. Above 1200 ºC, more complex
layered microstructures containing Al2TiO5, rutile, α-Al2O3 and continuous void layers
formed. The planar defects formed after Ti2AlC oxidation at 1200 ºC and Ti2AlN oxidation at
1100 ºC for 1h were identified as twins and stacking faults. After heating both Ti2AlC and
Ti2AlN to 1400 ºC for 1h and cooling to room temperature, cracks propagate in TiO2 grains.
Planar defects and cracks may arise from stress generation in the oxide scale. The thermal
stresses formed during cooling may result from thermal expansion mismatch of phases (TiO2,
Al2O3 and Al2TiO5) in the oxide scale, the high anisotropy of thermal expansion in Al2TiO5
and thermal expansion mismatch between the oxide scale and Ti2AlC or Ti2AlN substrate.
Growth stresses formed during isothermal oxidation treatment may arise from the volume
changes associated with oxidation reactions of Ti2AlC or Ti2AlN.
An oxidation mechanism for Ti2AlC is proposed, in which the growth of oxide scale is
caused by inward diffusion of O2- and outward diffusion of Al3+ and Ti4+. The weakly bound
Al leaves the Al atom plane in the layered structure of Ti2AlC, and diffuses outward to form a
protective inner α-Al2O3 layer between 1100 and 1300 ºC. However, the α-Al2O3 layer
becomes cracked at 1400 ºC, providing channels for rapid ingress of oxygen to the body,
leading to heavy oxidation. An oxidation mechanism for Ti2AlN is proposed, which involves
initial reaction with atmospheric oxygen to form oxide phases, demixing of the mixed oxide
phases, and void formation due to the Kirkendall effect and gaseous NOx release. The
oxidation resistance of Ti2AlC (up to 1400 ºC) is better than that of Ti2AlN (up to 1200 ºC).
Stress generation and gas formation appear to play important roles in the different oxidation
mechanisms of Ti2AlC and Ti2AlN.
Microstructural development during high-temperature oxidation of Ti3AlC2/W composites
involves α-Al2O3 and rutile formation ≥1000 ºC and Al2TiO5 formation at ~1300 ºC while
tungsten oxides may have volatilised above 800 ºC. Likely due to exaggerated, secondary
grain growth of TiO2-doped alumina, fine (<1 μm) Al2O3 grains formed dense, anisomorphic
laths on Ti3AlC2/5wt%W surfaces ≥1200 ºC and coarsened to large (>5 μm), dense, TiO2-
doped Al2O3 clusters on Ti3AlC2/10wt%W surfaces ≥1400 ºC, which were more protective
than well-dispersed Al2O3 grains. W may affect the oxidation behaviour of Ti3AlC2/W
composites in two ways: a) beneficially by weakening the Ti-Al bond in Ti3AlC2 by
attracting Ti to form Ti1-xWx resulting in a higher diffusivity of Al which diffuses outward to
form a protective α-Al2O3 layer during high-temperature oxidation; and b) detrimentally by
releasing volatile tungsten oxides so generating pores in the oxide scale. However, at high
temperature (≥1400 ºC) the former beneficial effects appear to be dominant over the latter
detrimental effect.
Date Issued
2011-10
Date Awarded
2011-12
Copyright Statement
Attribution NoDerivatives 4.0 International Licence (CC BY-ND)
Advisor
Lee, Bill
Sponsor
Lee Family Scholarship
Creator
Cui, Bai
Publisher Department
Materials
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)