Marketed products commonly require some type of marking on the product for commercial, regulatory, cosmetic or functional purposes. Desirable attributes for marking include consistent appearance, durability, and ease of application. Appearance refers to the ability to reliably and repeatably render a mark with a selected shape, color and optical density. Durability is the quality of remaining unchanged in spite of abrasion to the marked surface. Ease of application refers to the cost in materials, time and resources of producing a mark including programmability. Programmability refers to the ability to program the marking device with a new pattern to be marked by changing software as opposed to changing hardware such as screens or masks.
Anodized metal articles, which are lightweight, strong, easily shaped, and have a durable surface finish, have many applications in industrial and commercial goods. Anodization describes any one of a number of electrolytic passivation processes in which a natural oxide layer is increased on metals such as aluminum, titanium, zinc, magnesium, niobium or tantalum in order to increase resistance to corrosion or wear and for cosmetic purposes. These surface layers (also known as “anodic oxide layers”) can be colored or dyed virtually any color, making a permanent, colorfast, durable surface on the metal. Many of these metals can be advantageously marked using aspects of the instant invention. In addition, metals such as stainless steel which resist corrosion can be marked in this fashion. Many articles manufactured out of metals such these as are in need of permanent, visible, commercially desirable marking. Anodized aluminum is an exemplary material that has such needs.
Creating color changes on the surface of anodized aluminum articles with laser pulses has been known for several years. An article titled “Dry laser cleaning of anodized aluminum” by P. Maja, M. Autric, P. Delaporte, P. Alloncle, COLA'99—5th International Conference on Laser Ablation, Jul. 19-23, 1999, Göttingen, Germany, published in Appl. Phys. A 69 [Suppl.], S343-S346 (1999), pp S43-S346, describes removing anodization from aluminum surfaces, however, note is taken of color changes which occur at laser energies below that required for removal of anodization from the surface.
One mechanism which has been put forth to explain the change in optical density or color of metallic surfaces is the creation of laser-induced periodic surface structures (LIPSS). The article “Colorizing metals with femtosecond laser pulses” by A. Y. Vorobyev and Chunlei Guo, Applied Physics Letters 92, (041914) 2008, pp 41914-1 to 141914-3 describes various colors which may be created on aluminum or aluminum-like metals using femtosecond laser pulses. This article describes making black or gray marks on metal and creating a gold color on metal. Some other colors are mentioned but no further description is made. LIPSS is the only explanation offered for the creation of marks on metallic surfaces. Further, only laser pulses having temporal pulse widths of 65 femtoseconds are taught or suggested to create these structures. In addition, no mention is made as to whether the aluminum samples are anodized or have had the surface cleaned prior to laser processing. Further the article does not discuss possible damage to the oxide layer.
When discussing laser pulse duration, the method of measuring pulse duration should be defined. Temporal pulse shape can range from simple Gaussian pulses to more complex shapes depending upon the task. Exemplary non-Gaussian laser pulses advantageous for certain types of processing are described in U.S. Pat. No. 7,126,746 GENERATING SETS OF TAILORED LASER PULSES, inventors Sun et al., which patent has been assigned to the assignees of the instant invention and is hereby incorporated by reference. This patent discloses methods and apparatus to create laser pulses with temporal profiles that vary from the typical Gaussian temporal profiles produced by diode pumped solid state (DPSS) lasers. These non-Gaussian pluses are called “tailored” pulses because their temporal profile is altered from the typical Gaussian profile by combining more than one pulse to create a single pulse and/or modulating the pulse electro-optically. This creates a pulse which the pulse energy varies as a function of time, often including one or more power peaks wherein the instantaneous power increases to a value greater than the average power of the pulse for a fraction of the pulse duration. This type of tailored pulse can be effective in processing materials at high rates without causing problems with debris or excessive heating of surrounding material. An issue is that measuring the duration of complex pulses such as these using standard methods typically applied to Gaussian pulses can yield anomalous results. Gaussian pulse durations are typically measured using the full width at half maximum (FWHM) measure of duration. In contrast to this, using the integral square method, as described in U.S. Pat. No. 6,058,739 LONG LIFE FUSED SILICA ULTRAVIOLET OPTICAL ELEMENTS, inventors Morton et al., allows complex pulse temporal shapes to be measured and compared in a more meaningful manner. In this patent, pulse duration is measured using the formula
  t  =                    (                  ∫                                    T              ⁡                              (                t                )                                      ⁢                          ⅆ              t                                      )            2              ∫                                    T            2                    ⁡                      (            t            )                          ⁢                  ⅆ          t                    where T(t) is a function which represents the temporal shape of the laser pulse.
Another problem with reliably and repeatably producing marks with desired color and optical density in anodized aluminum is that the energy required to create very dark marks with readily available nanosecond pulse width solid state lasers is enough to cause damage to the anodization, an undesirable result. “Darkness” or “lightness” or color names are relative terms. A standard method of quantifying color is by reference to the CIE system of colorimetry. This system is described in “CIE Fundamentals for Color Measurements”, Ohno, Y., IS&T NIP16 Conf, Vancouver, Conn., Oct. 16-20, 2000, pp 540-545. In this system of measurement, achieving a commercially desirable black mark requires parameters less than or equal to L*=40, a*=5, and b*=10. This results in a neutral colored black mark with no visible grayness or coloration. In U.S. Pat. No. 6,777,098 MARKING OF AN ANODIZED LAYER OF AN ALUMINIUM OBJECT, inventor Keng Kit Yeo describes a method of marking anodized aluminum articles with black marks which occur in a layer between the anodization and the aluminum and therefore are as durable as the anodized surface. The marks described therein are described as being dark grey or black in hue and somewhat less shiny than unmarked portion using nanosecond range infrared laser pulses. In addition, the aluminum is required to be cleaned of all surface particles, for instance particles remaining after polishing, prior to anodization. Making marks according to the methods claimed in this patent are disadvantageous for two reasons: first, creating commercially desirable black marks with nanosecond-range pulses tends to cause destruction of the oxide layer and secondly, cleaning of the aluminum following polishing or other processing adds another step in the process, with associated expense, and possibly disturbs a desired surface finish by further processing.
What is desired but undisclosed by the art is a reliable and repeatable method of making marks on anodized aluminum in either black, white or grey levels in between or in color that does not require an expensive femtosecond laser or disturb the oxide layer in the process or require cleaning following surface preparation. In addition, no information is supplied on how to repeatably create various colors on anodized aluminum surfaces, nor has the effects of bleaching or damage to the anodization layer been thoroughly investigated. What is needed then is a method for reliably and repeatably creating marks having a desired optical density or grayscale and color on anodized aluminum using a lower cost laser, without causing undesired damage to the overlaying oxide or requiring cleaning prior to anodization.