Titanium adhesive bonding is an engineering process used in the aerospace industry, medical-device manufacture and elsewhere. Titanium alloy is often used in medical and military applications because of its strength, weight, and corrosion resistance characteristics. In implantable medical devices, titanium is used because of its biocompatibility and its passive, stable oxide layer. Also, titanium allergies are rare and in those cases mitigations like Parylene coating are used. In the aerospace industry titanium is often bonded to save cost, touch times, and the need for mechanical fasteners. In the past, Russian submarines' hulls were completely made of titanium because the non-magnetic nature of the material went undetected by the defense technology at that time. Bonding adhesive to titanium requires preparing the surface beforehand, and there is not a single solution for all applications. For example, etchant and chemical methods are not biocompatible and cannot be employed when the device will come into contact with blood and tissue. Mechanical surface roughness techniques like sanding and laser roughening may make the surface brittle and create micro-hardness regions that would not be suitable for cyclic loading found in military applications. Air oxidation at high temperatures will produce a crystalline oxide layer at a lower investment cost, but the increased temperatures can deform precision parts. The type of adhesive, thermosetting or thermoplastic, and curing methods are also factors in titanium bonding because of the adhesive's interaction with the treated oxide layer. Surface treatments can also be combined. For example, a grit blast process can be followed by a chemical etch and a primer application.
Abrasives Aluminium oxide or Alumina and Silicon carbide are most commonly used to prepare titanium for epoxy bonding. Alumina has a hardness of 9 on the Mohs scale while silicon carbide has a hardness of just under that of diamond. Alumina particle sizes in the 10 to 150 micron range are used depending on the workpiece geometry and blasting capabilities. Silicon carbide particles are typically in the 20 to 50 micron range with texturing occurring at a faster pace than alumina. When silicon carbide hits the titanium surface the operator will see sparks as is common with titanium surfaced golf drivers when they hit the ground surface. Care must be taken if sensitive electronic assemblies are housed within the titanium enclosure. Electrostatic discharge can be mitigated with point ionizers or grounding features in the tools. Glass beads media are used less commonly. They come as spherical particles in the 35-100 micron range. They are a 6 on the Mohs scale and are oftentimes used with water to create a hydrohone slurry. When applied to commercially pure titanium material they will stress relieve the assembly, typically after welding, and create a satin-like finish perfect for laser marking of labels. The surface is also suitable as preparation for assemblies prior to vapor deposition of Parylene coating.
Surface roughness is achieved through the use of a blasting nozzle propelled by compressed air. The focus and velocity of the media created by the nozzle can be varied depending on the roughness requirements and repeatability. Surface roughness measured using Ra, Sa and Sdr is used to characterize the media application and the adhesive bonding strength. Typical Ra values for commercially pure titanium are between 0.2 and 0.75 micro meters. The surface roughness can be tailored to the epoxy viscosity and curing conversion. The roughened surface is rinsed with process water or an alkaline cleaner and is often sealed with a primer application like Silane A-187 or alkoxide. Application of the primer can be achieved through manual means, like a brush. It can also be sprayed on the roughened surface or the whole assembly can be dipped in a primer solution and cured. On commercially pure titanium surfaces that have been roughened with silicon carbide, a silane primer will darken the surface allowing for verification of application. Implantable medical devices are often manufactured in a cleanroom environment. Typical cleanroom ratings are within the ISO-7 and ISO-8 range or between class 10k and 100k. Abrasives and their application cannot be housed in such cleanrooms. If pass through windows are not available then laser roughening is a good option.
Laser roughening
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