55
Aconsideration on the Carburized Layer of Ceramic Tube
i ISHIIxi， Kαzuposhi YAMANAKA＊＊，
Tadashi AIHARA＊R・＊
Summary
The effect of carl）urizing of the ceramic tubes on their mechanical properties， microstructure and X−ray characteristics were studied． NC type and SSA−H type ceramics are carburized into black phase， similar to steel cementation． The strength and elastic modulus decrease with the thickness of carburized la｝・er． Microstructure shows the carbon penetration into the porosity and grain boundary， however， no remarkable crystaUographic changes are observed in this layer．
IIltroduction
Actually， there is no treatment like carburizing of ceramics， without the case when accidentally ceramic tubes are carburized in the process of cementation for
steels． From this reason there is no systematic technical information1）・2）about this phenomenon．
At五rst we found the color change of NC type tube by carburizing， then we tried
to carburize some other ceramics． By carburizing， some properties of ceramic tubes change apparently． For example， the color of surface turns to black from white，strength decreases， and ductility and surface electrical conductivity increase．
We try to丘nd out here the fundamental carburizing mechanism and the property changes by carburizing． And also discussion will be made on the effect of chemicaI
composition and the result of X−ray diffraction．
Experimental procedure
Four typical ceramic tubes are chosen for carburizing， those are NC type， SSA−
Htype， SSA−S type and silica． Nominal chemical compositions are listed in Table 1．
These ceramic tubes are carburized and thell suffered for various testing． The
carburizing conditions are as follows；charcoal 60％， BaCO330％and NaCO310％
at 1060℃for maximum 96 h， these conditions are similar to that of steels except temperature．
The carburized Iayers are observed by optical microscope and X−ray diffraction ＊理工学部機械工学科教授 金属材料学
＊＊理工学部機械工学科助教授 材料力学
＊＊＊理工学部機械工学科教授 工業材料学
type
Sio2， wt％
Al203， wt％
denslty
NC
49 47
2．5
SSA−H
3 95．4
3．7
SSA．S
0．1
99．5 3．9
silica
93
5 2． 5
apparatus． X−ray apparatus is made by Rigakudenki with Cu target by l kW． The X−ray specimens are ground into fine powder and then diffractometer is used for identification． The mechanical properties are tested by Shimadzu universal testing machine with capacity of 2 tons． The strength and ductility are measured by
compression test using ring shape test pieces， refering other bend test4）．
．Results and discussion
We tried to carl〕urize all four types of ceramics． However， silica and SSA−S type
（alumina）were not carburized， but the color of surface changed slightly to gray．
On the contraty， NC type and SSA−H type had quite different behaviors， and these were carburized exactly with similar manner of steel cementation5）， ill the respect of surface color change to black， even into the core of materials． Therefore only
NC type and SSA−H type wi11 be dealed with mainly．The boundary between original and carburized phase is very clear， while they
have different colors． In Fig．1the illustrated fracture surface of carburized specimen is shown． It is seen that the outer layer has dark color and inside white area is
original one． The thickness of carburized layer naturally depends oll the time of treat皿ent， as shown in Fig．2． It shows that the carburizing of ceramic proceeds monotonously until full carburization．、 And also it shows that NC type ceramic tubes will have l mm thickness of carburized layer after 96 h by this condition，however it may take twice for SSA−H type tube．
Fig．1 111ustration of fractured surface
of carburized NC type ceramics．
1．0
0．75
宕
ε缶
0．5
8
・
讐
B 5
鳶響
iO25
0
Carburizing Time（h）
Fig．2 Carburizing curves for NC and SSA−H type ceramics．
SSA−H type NC type Fig．3 Dimensions of compression test pieces．
The dimensions of ring shape test piece for compression test are shown in Fig．3．
Fig．4shows the microstructure of carburized Iayer of NC type and SSA−H type tube． These ceramic tubes are used as a component of vacuum equipment． Even
for tllese air tight structure， they should have porosity， cavity， vold or vacancy．
Actually in the case of NC type many porosities are observed， and it has very五ne
crystal grains． And also SSA−H type ceramics has丘ne grains．The mechanical property of ceramics after carburizing is one of the most important
ドロ ドぐ
切
疋
の
聡
4 Microstructure of carburized layer of ceramic tubes b） SSA−H type ◆
⌒
7 UtEこ〇三〇忘言8言8WtlS ean︸D？Jd
Fig．5
Fracture stress vs carburizing time．
aspects for this phenomenon． The compression test has been carried out for four
Ievels of carburized specimen． As mentioned before， the tube carl〕urized for 96 h has l mm carburized layer． The strength vs carburizing time is shown in Fig．・5．
The strength is going down with carl〕urizing time to 1／3〜1／60f original strength，
that means the carburized layer is not available for strength． However the thickness of Iayer may not play an important role．
In Fig．6the tendency of elastic modulus with carburizing time is shown． The
value of elastic modulus represents deformability or a sort of ductility． The ceramic
z （ εEこSn︶ snlnpou．s D1lselH
Carburizing time （h）
F三g．6 Elastic modulus vs carl）urinzing time．
material usually shows very low ductility， therefore it is remarkable that the elastic
modulus decrease w三th similar manner of strength． However， the total amount of deformation， which should be c皿sidered for the real ductiIity of these materials，was very small even for the carburized specimen．
Many small flalK・s are observed at the surface of the carburized layer， so that it may be oIle of the reason for the decrease of strength for carburized specimen． If the initiation of surface crack can be prevented， there is a chance to get some kinds of ductile ceramics．
Results of X・−ray diffraction show that the silica tube， opaque type， has a amor・
phous phase． SSA−S type and SSA−H type have been identi丘ed as Al203 as expected．
However NC type has quite different mullite phase． Also it is very clearly shown
that the existlng phase does not receive any influence from the carburizing treatment，
that is indicated by the X・−ray studies where the diffraction peaks of both speci mens， before and after carburizing， have exactly the same peaks at the same angles．
From these results， the color change of ceramic tubes does not mean any essential
phase transformation． Then carl〕on penetrates only into the porosity or the bo皿．dary layer of original sintered particles． The reason for weakening of ceramic tubes
should be considered that these penetrated carbon should play a role as defect of oxide grains． And for more accurate discussion of the weakening mechanism andthe decrease of elastic modulus， further microscopic phase analysis should be needed．
However， most ceramic tubes have the demerit of fragility． If these carburizing
treatments can increase ductility， it皿ay have some applications in future．
The electric conductivity was measured on the carburized specim飽， however it does not show clear conductivity， only very thin surface工ayer may have some
conductivlty， but not essential one．
Conclusions
Four types of ceramic tubes are tried in carburizing treatment as the similar
by carburizing， however NC type and SSA−H type tubes show very clear carburized
layer． The thickness of layer is about l mm after 96 h treatment at 1060℃． The strength of carburized tube drop to 1／3〜1／6 after 96 h carburizing． Also the elastic
modulus is going down with the carburizing， that means， the ductility may be improved by this treatment． Carburized layer does not have particular phase， so it may have only carbon penetrated layer．Acknowledgements
We appreciate for that we could use the X−ray apparatus of Chemical Department and for the丘nallcial support of Special Grant by Meisei University． This paper is apart of graduation thesis bY senior students of Oppata， Takayanagi and Matsuoka．
