| Sapphire is desired in many high-tech areas.In fabrication of large-sized or complicated structure sapphire joints,joining technology is one of the key technologies.The currently most widely used welding methods for sapphire require high environmental temperature,which usually causes significant thermal stress problem.If the metal/sapphire bonding could be achieved at a lower temperature,it will very useful in simplifying the manufacturing procedure,promoting effectiveness and lowering the cost.The low temperature will be very beneficial in controlling thermal stress in the joints.To date,some research have indicated that bonding between A1 and sapphire could be achieved at a reduced temperature.However,partial bonding can be achieved by using the existing methods.A special physical environment can be made at solid/liquid interface by using ultrasonic methods,so that many reactions take place in an environment that they usually do not.This thesis mainly aims revealing the effects of ultrasound in promoting epitaxial reaction at liquid alloy/sapphire interface.And then,basing on these effects,novel methods for brazing sapphire at a medium or low temperature were investigated.Thus a new technical route to joining sapphire using epitaxial reactions in medium-low temperature was proposed.In the first step,the characteristics of the epitaxial reaction at sapphire/pure-Al and sapphire/Al-Al-4.5Cu-1.5Mg alloy interfaces was investigated.In these two alloys the sapphire plates were sonicated at 700℃.As the sonication increased from zero to 300s,the products at sapphire/pure aluminum interface turned from a thin amorphous layer into a nano-crystallineγ-Al2O3 layer.Then,when the sonication was 1000s,the products turned in to a epitaxial α-Al2O3 layer.In this procecdure,both growth in thickness and phase transformation occurred at the interface.The production at the sapphire/Al-Al-4.5Cu-1.5Mg alloy interface was MgAl2O4 spinel,which was heterogeneous epitaxy.In liquid Sn-Zn-Al alloy,sapphire plates were sonicated at 250℃.The interfacial reaction products turned form amorphous into a mixture of amorphous and nano-crystalline alumina layer.A quasi-epitaxial lattice relation between the interfacial reaction products and the sapphire substrate was found.Full coverage of reaction on the sapphire surface can be made.Based on these,sapphire joining method with Al-based,Zn-based and Sn-based alloy at medium to low temperature was designed and applied.The design principles were:firstly,the alloy shall contain some Al so that robust bonding at the sapphire/alloy interface could be achieved;and secondly,thermal stress in the joints made with Al and Zn-based alloy shall be mediated.Pure aluminum was used for joinig sapphire,while the shear strength of the joints was only～50MPa.Al-12Si alloy has higher strength than pure aluminum but shear strength of the the joints made with it was only～60MPa.The most vulnerable place in the joints was the Si/sapphire interface.Al-4.5Cu-1.5Mg alloy has very high strength and contains no Si.However,when it was used as filler materials,significant thermal stress problem occurred,inducing cracks at the metal/sapphire interface and the shear strength was only～60MPa.A heat treatment for stress relief were used during the cooling,which eliminated the cracks at the interface.Thus,a nominal shear strength of～130MPa was achieved.Finite element method(FEM)was used to estimate the shear strength and tensile strength at the interface were not less than 230MPa and 190MPa,respectively.Zn-4Al alloy was used as filler material.Interfacial reaction and deposition of an alumina layer on sapphire were realized at 420℃.However,many cracks existed at the metal/sapphire interface when sapphire joints were made with the Zn-Al alloy.The shear strength was only～60MPa.Therefore the abnormal absorption of the Zn-Al alloy to the matrix of 55%SiCp/A356 composites materials were used.Composites were cut into precursor interlayers and placed in the brazing seam.A new Zn-Al based composite structure joint was made,and the volume fraction of particles in the joints reached as high as 40%.And the estimated CTE of the joints was 7.4×10-6/K,which was very close to that of sapphire.The shear strength of the joints reached up to 155MPa。Self-made Sn-3Ag-0.5Cu-1Al,Sn-9Zn-2Al and Sn-53Bi-0.5Al alloy were used to solder sapphire joints.The highest shear strength was 57MPa,48MPa and 27MPa,respectively.When the alloys does not contains Al,they could spread on sapphire in the ultrasound field,but bonding between the alloys and the sapphire substrate was a physical-attachment type,with hardly any strength.When Al was added into the alloys,interfacial nano-alumina layer was made in the ultrasonic dipping,and a Al-rich layer formed at the sapphire/alloy interface.These two effects worked together to increase the interfacial strength. |
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