“The interference of light reflected through the oxide layer that forms on it gives titanium its colour.”
After gold, titanium is the most cherished metal in the world. It is used for a wide variety of things, from essential things like surgery and spacecraft to just decorative things like jewellery and keychains.
This adaptable yet esoteric metal oxidises one of the most exquisitely, producing mesmerising hues and finishes that are unmatched. What then imparts this grey, plain metal’s attractive appearance?
Titanium – the metal, the myth, the legend?
Despite its low density and high corrosion resistance, titanium is renowned for its superior mechanical qualities, which include remarkable strength. Although titanium is not regarded as a precious or rare earth metal, its extraction from its ore requires a lot of resources, hence it is extremely expensive.
Due to its mechanical characteristics, it is highly challenging to deal with during operations like fabrication and machining, which raises the price of the finished product. Since the method of obtaining titanium hasn’t evolved much throughout time, using this abundant metal is still quite difficult.
Titanium oxide – The cause of color
Metals and oxygen aren’t exactly best buddies, if iron’s example is any indication. Titanium is an exception to this rule. Titanium oxidises to create an almost impenetrable barrier by the use of both heat and electricity. Numerous noteworthy characteristics of this barrier include mechanical rigidity and optical deception.
The oxide layer becomes thicker with increasing temperature or voltage. In contrast to rust, titanium oxide can be difficult to remove mechanically, obscuring the new layer of titanium beneath.
Perception of color – Thin film interference
Changes in chemical composition or the addition of pigment cannot be the cause of any of the titanium’s oxidation-related colouring. In actuality, the oxide layer on the human eye is pulling a prank on us.
Since the oxide layer is transparent and thin, light reflects off of both its top and bottom surfaces. Because of the interference between the reflected light beams, some light wavelengths are suppressed while others are amplified to produce vibrant hues. Thin film interference is another name for this phenomena.
The oxidation layer thickens and stabilises with increasing heat or voltage. Both discrete and continuous hues are produced as a result of the change in refractive index. Even after heat or voltage are turned off, titanium retains its colour unless it is manually removed.
Oxidizing titanium – Heat
Historically, heat has been used to oxidise titanium. Heat oxidising can be a difficult procedure to complete since it is physically impossible to control the flow of heat within the metal. The removal of oxide layers from undesirable locations necessitates post-processing in addition to specific equipment for even heating.
Such processing makes use of techniques including selective sand blasting and mechanical abrasion, such as sanding.
As the temperature rises, the oxide layer’s thickness does as well. As a result, different temperature ranges are related to different colours of titanium oxides.
|Temperature ( °C)||Color of oxide layer|
|385||Pale gold straw|
Glossy finishes are obtained when titanium work pieces are heated.
Oxidizing titanium – Voltage
When titanium is immersed in a conducting substance, it can also be heated to create an oxide coating on it (electrolyte). Anodizing is another name for this. The negative terminal of the battery can be attached to any other metal, such as aluminium, copper, or stainless steel, while the positive terminal of the battery is connected to the titanium work piece.
For a specific voltage, the oxide film’s thickness doesn’t change. As a result, different hues will correspond to different voltages. However, the thickness can be adjusted by altering the electrolyte’s composition.
Thinner oxide layers, typically with a range of only a few nanometers, are known to be produced by acidic and neutral electrolytes. However, it is known that powerful alkaline media can create coatings as thick as several microns.
|Applied voltage (V)||Color of oxide layer|
When making coloured titanium oxides, there are several benefits to employing electricity as opposed to heat. While selective dipping and masking allow only the appropriate sections of the work piece to get coloured, controlling the flow of electricity is possible.
A more practical alternative to heat oxidising titanium is anodizing. This is due to the fact that giving an exact and equal amount of heat is more difficult than supplying the voltage needed to produce a specific colour.