ReviewChemical composition, microstructure and sintering
temperaturemodications on mechanical properties of TiC-based cermet
A reviewA. Rajabia, M.J. Ghazalib,, A.R. DaudaaSchool of Applied
Physics, Faculty of Science and Technology, National University of
Malaysia (UKM), 43600 UKM, Bangi, Selangor, MalaysiabDept. of
Mechanical & Materials Engineering, Faculty of Engineering
& Built Environment, National University of Malaysia (UKM),
43600 UKM, Bangi, Selangor, Malaysiaarti cle i nfoArticle
history:Received 12 June 2014Accepted 28 October 2014Available
online 11 November 2014Keywords:CermetsTiCTiNSinteringMechanical
propertiesabstractCermets, particularlythosebasedonTiC,
arereceivingconsiderableattentionbecauseoftheiruniqueproperties,
including high hardness and resistance to thermal deformation.
However, TiC-based cermetslacksufcienttoughness.
Toimprovetheperformanceofthesecermets,
numerousstudieshavebeenconducted to determine the factors that can
be manipulated to improve their toughness. However, theresults of
these studies vary. This paper summarizes the studies to improve
cermet design via chemicalcompositions and microstructures.
Critical issues including the effects of grain size and sintering
temper-ature on the mechanical properties (i.e., toughness,
hardness, and wear resistance) of TiC-based cermetsare also
discussed. 2014 Elsevier Ltd. All rights reserved.1.
IntroductionSince 1927, various materials have been investigated
toimprove tool life and mechanical properties of cermets and
cuttingtools that are extensively utilized; roughly two
billion-dollar allot-ment in semi-nishing and nishing works have
been allotted forpolymers, ferrous alloys, nonferrous alloys, and
advanced materialssuch as intermetallics and composites [13].
Cermets present ben-ecial resistancetooxidationthroughout metal
machiningandbuildupedgeformationtowingtotheirgoodchemicalstabilityand
high-temperature hardness [46]. In general, cermets includetwo
different phases (i.e., ceramic and metal binder). The
ceramicpartsretainthehighhardnessandcuttingability,
butthesofterphase (binder) can deform and absorb energy [79]. The
ceramicsare compounds of carbon, nitrogen, oxide, boron, and
silicon withmetals such as tungsten, titanium, tantalum, niobium,
aluminum,and molybdenum [4,1012]. Among those, carbides are very
stiffand have high melting temperature, ranging from2000 C to4000
C, andtheirhardnessissecondtothatofdiamond, whichis the hardest
known material [1315].Thus, titanium carbide(TiC),
withacubicstructure similartothatofNaCl,
hasbeenconsideredasoneofthemostsignicantmetal
carbidesformanufacturinganewgenerationof
cermets.Suchcermetsmaybeendowedwithintrinsicpropertiesof
TiC,namely, low density (4.93 g/cm3), low friction coefcient,
thermalstability, highsolvencywithothercarbides,
highmeltingpoint(3067 C), high elastic modulus (410450 GPa), high
thermalconductivity(30 106 S/cm), as well as higher
thermodynamicstability and hardness than WC (33% more than WC)
[1620].Conventionalcermets(WCCo)aretoughandhard;however,these
materials possess poor oxidation resistance and plasticdeformation
at high temperature, which prohibit their applicationat elevated
temperatures [2124]. Asa result,a growing interesthas been given to
the use of TiC-based cermets because of their sev-eral
advantages[4,25]. However, usingTiC-basedcermetsinitsearly stage is
not advisable because of their inherent brittleness
atlowtemperature [26,27]. Thus, scholars have attempted to
improvethe mechanical properties of TiC-based cermets [24,28,29].
Ingeneral, the mechanical properties of cermets depend on
chemicalcomposition and sintering temperature [3032]. Thus, the
presentstudy aimed to summarize the current state of knowledge
concern-ing the effects of chemical composition, microstructure,
and sinter-ing temperature on the mechanical properties of
TiC-based cermets.2. Chemical composition2.1. TiNThe use of TiN is
constantly developing because of its prominentphysical and chemical
properties, such as high meltingtemperature(2927 C),
relativelylowoxidation, andlowfrictioncoefcient.
TiNcanbeutilizedashardcoatingandanessentialmaterial for cermet
andcutting tools owing to its highwearresistance
[3335].http://dx.doi.org/10.1016/j.matdes.2014.10.0810261-3069/
2014 Elsevier Ltd. All rights reserved.Tel.: +60 3 89216418; fax:
+60 3 89259659.E-mail addresses: [email protected],
[email protected](M.J. Ghazali).Materials and Design 67
(2015) 95106ContentslistsavailableatScienceDirectMaterials and
Designj our nal homepage: www. el sevi er . com/ l ocat e/ mat
desSince 1970, several studies have been conducted to improve
thetoughness of TiC-based cermets by modifying their
microstructureand grain size [36,37]. Changing the chemical
composition of TiN isacrucialstrategy,
andTiNhasbeenconsideredmorethanotherelements. The hardness of TiNis
muchless thanthat of TiC(TiC = 32 GPa; TiN = 20 GPa); thus, TiNcan
effectively improvethe toughness of TiC. However, the hardness and
Youngs modulusof TiC are reduced, as shown in Table 1
[38,39].Another important issue to be noted is the quantity of
TiNaddedin the TiC-based cermets. Moskowitz et al. [40]
demonstrated thatthe maximumtool life value (TiCTiN22.5Ni10Mo10VC)
isachieved when the R = TiN/(TiN + TiC) ratio is approximately
45,as illustrated in Fig. 1, because of grain renement and
dissolutionof Mo in binder phase, which leads to solid solution
hardening. Inthiscase, thefracturetoughness(KIC) isdecreasedwhenR
> 5,resulting in a decreased tool life because of the formation
of nergrains. Although the effect of R ratio [TiN/(TiN + TiC)] on
mechanicalproperties of the cermets was investigated [40], the main
reason ofthe decrease in grain size is not signicantly considered.
Asdescribedinseveral studies[4143], typical microstructures
ofTiC-based cermets involve a rim that encompassed the whole
core,as shown in Fig. 2. In general, dissolution and
re-precipitation of themetallic carbide (Ti, W, Mo, Ta) on the
cores in the binder phaseresults in rim formation [41]. Due to the
complexity of the corerim formation ofthecermets, afairassessment
oftheformationmechanismsremainadifculttask[38]. Recently,
manystudieshave been conducted to understand the mechanisms of
corerimformation in cermets. Solid solution of rim, such as that in
Ti(CN),can be made through partial replacement of C atoms with
NbecauseTiN decomposes at elevated temperature [42,43]. In this
case, a dis-solutionreprecipitationprocessis
limitedduetothedecliningamount of TiC that is dissolving in the
liquid phase, which resultsin decrease in grains and moderate rims.
As Ling et al. [42] observedthat the average grain size of the
cermets decreases from1.44 lmto0.78 lmwith increasing TiNaddition
from0 to 37 wt.%, as shown inFig. 3; however, this nding is
controversial. Howrimthickness canbe hindered by existence of TiN
particles in the cermets at duringsintering has not been
elucidated. Liang et al. [42] found that N
inthecermetscanactasanobstaclethatiscapableofpreventingthe
diffusion of Mo into rim at temperatures above 900 C; hence,this
process can be effectively employed to decrease rim thickness.If
these conditions are met, then the toughness of the cermets
canbeimproved, probably because of tensile stress reduction at the
coreriminterface [43]. Tensile stress promotes crack propagation
withina corerim interface, leading to a decline in toughness
[44].Previous studies [36,45,46] observed that microstructures
withcoarsegrainsprovidehigherKICthannergrains,
inwhichthetranscrystalline fracture of the coarse grains offer a
higher bondingforce than the intergranular fracture of ne grains.
In general, thenergrainsofalloysassistintheincreaseinhardness,
resultingin a decline in KIC, as described by the following
equation [47,48].KIC 2:15 106EH 0:61 0:012EH 0:6H1:51where H, E,
and KIC are the hardness, Youngs modulus, and fracturetoughness,
respectively.By contrast, Sun et al. [49] observed an increase in
toughness inWC11Co cermets with the formation of ne grains, which
may beinducedbyincreasingthedensity. Theformationofnergrainsresults
in a decline in pore size and an increase in fracture tough-ness.
Such changes can be explained by the existence of cracks thatcan be
easily propagated frompores during fracture process[50,51].
Abriefcomparisondemonstratesthatpoordensicationcan be derived from
large particles, whereas high densication isobtained by nano-grains
to create large surface areagrain bound-ary [5254]. Consistent to
this, the physicalmechanical propertiesof materials with the same
chemical composition can be improvedusing nano-grains [5558]. This
hypothesis can be approved by theHallPatchrelationship[59,60].
Previousstudies[32,61]showedthat the presence of nano-TiN (10 wt.%)
in TiC cermets
provideshigherfracturetoughnessandtransverserupturestrength(TRS)than
that of WCCo cermets (Table 2). Such improvement isbelieved to be
due to the existence of nano-TiN in grain boundarythat acts as an
obstacle for grain growth; nano-TiN do not
dissolveinthesinteringprocesscompletelyanddonot
diffuseintotheceramic hard phase. Moreover, nano-TiN sharply
hinders the coa-lescence of TiCgrains
whendistributedbetweenTiCparticles(Fig. 4), resulting in disrupted
motion of dislocations. Thus, grainsize reduction (renement) in the
cermets can improve mechanicalproperties such as TRS, based on
HallPetch formula [62]. A previ-ous study[32] indicatedthat
intergranular fracture(alongthegrain boundaries) is usually the
dominant failure mode. Thus, crackpropagation can be limited by
nanoparticles of TiN that are distrib-uted in grain boundaries of
TiC [63]. The main reason for the occur-rence ofsuch phenomenon
isthat cracksgrowing alongtheTiC/TiNnanoneedmoreenergytopropagate,
resultinginimprovedmechanical properties. Therefore, the
distribution of TiN nanopar-ticles between TiC particles or inlayed
between particles can be asignicant factor [32,61]. In addition,
Han et al. [32,61] indicatedFig. 1. Tool life of AISI 4140 steel at
158 m/min as a function of TiN-to-(TiC + TiN)ratio at sintering
temperature of 1400 (s), 1475 (d), and 1550 C (N) [40].Fig. 2.
FE-SEM image of sintered cermet by SPS at 1718 K [32].96 A. Rajabi
et al. / Materials and Design 67 (2015) 95106that some added TiN
nanoparticles (
