Адгезия покрытий: почему высота пиков сама по себе не определяет прочность сцепления

Ask most specifications what a surface profile needs to be, and you get one number: peak height, usually in µm or mils. It is the most commonly measured and reported parameter for a blasted or profiled surface, but on its own it does not fully describe how well a coating will bond. This article explains the two profile parameters that matter, how to measure them, and what that means for writing and inspecting surface preparation specifications.

The two numbers that matter

Coating adhesion is better described as a function of two parameters together: peak height (H) and peak density (Pd), written simply as A = f(H, Pd). Peak height tells you how tall the anchor points on a profiled surface are. Peak density tells you how many of them there are per unit area. A surface can have a high peak height and still bond poorly if those peaks are sparse and irregular; conversely, a lower, denser, more uniform profile can outperform a taller but patchy one. Profiled surfaces are irregular by nature: peak height, sharpness and surface area cannot be fully captured by a single reported value.

Peak height

Peak height is the vertical distance from the bottom of the valleys to the top of the peaks. Field methods report it in different ways: replica tape and depth micrometers give a peak-to-valley value, while stylus instruments report roughness parameters such as Rt, Rmax or Rz. These are related but not identical, so a specification should always name the method along with the number. More background on profile depth is on our anchor profile page.

Peak density

Peak density is the number of peaks within a given surface area, typically reported in peaks per mm². The stylus equivalent, relative peak count (Rpc), is the number of peak and valley pairs per unit length that extend outside a small band around the mean line, typically reported in peaks per cm. Two surfaces with the same peak height can have very different peak densities, depending on the abrasive or tool, its condition and the operator's technique.

Why does peak density matter for adhesion?

A coating bonds to the surface area it wets. More peaks per unit area means more surface area and more anchor points for the coating. Research by Roper, Weaver and Brandon (2005), as summarised by KTA-Tator, found that "a high peak count characteristic of surface profile helps resist undercutting corrosion when the coating system becomes damaged while in service, and provides the coating system with better adhesion to the prepared substrate." More recent work by DeFelsko, also cited by KTA, likewise found that greater peak density promotes adhesion.

The former ASTM stylus method for peak count, ASTM D7127, put it the same way in its significance statement: optimising peak height and peak density can improve coating adhesion. That standard was withdrawn in 2021 and its content moved into ASTM D4417.

Why this matters for spec writers and inspectors

Two coatings applied to two surfaces with the same average peak height can perform differently in service if the peak density differs. For liquid-applied coatings, both a high-height/low-density surface and a low-height/high-density surface can, in principle, give an adequate bond. That is why good specifications combine a target profile range from the coating data sheet with a defined, repeatable preparation process and a measurement plan, rather than a single height reading taken in isolation.

• Name the method. Replica tape, depth micrometer and stylus readings are not interchangeable. State which method applies and how readings are averaged.
• Define the sampling. SSPC-PA 17 describes the frequency of measurements and the acceptance criteria for conformance to profile and peak count requirements.
• Include peak density where it is required. If the coating manufacturer or asset owner sets a peak count or peak density requirement, put it in the specification together with its measurement method.
• Control the process, not only the result. Abrasive type and size, recycling condition, tool settings and operator technique all change peak density. A process that is repeatable gives fewer surprises at inspection.

For the full structure of a profile clause, see how to write a surface preparation specification that gets results.

How are peak height and peak density measured?

• ASTM D4417 covers field measurement of surface profile, including the depth micrometer (Method B) and replica tape (Method C), and since 2021 the stylus method formerly in ASTM D7127.
• Stylus instruments trace the surface and report height parameters together with peak count (Rpc) per unit length.
• Optical grade replica tape with a suitable image-based reader measures both peak height and peak density from the same replica, which brings peak density into routine field inspection.

Our anchor profile measurement field guide explains how inspectors verify profile and consistency on site, and surface profiling equipment compares the tools that create and measure a profile.

What this means in practice

A repeatable mechanical process, rather than a manually variable one, is what keeps peak density consistent from pass to pass, not just peak height. That consistency is the premise of the MontiPower® Controlled Preparation System. On the Bristle Blaster®, the accelerator bar releases each bristle tip to strike and rebound, leaving a dense pattern of crater-like impressions rather than a smeared or polished surface. In documented tests this gives cleanliness comparable to ISO 8501-1 Sa 2½ / SSPC-SP 10 and a 65–85 µm Rz anchor profile on standard steel; results vary with steel grade, rust grade, belt condition and technique, so the profile should still be measured on every job. The texture behind those numbers was examined in a 3D optical analysis of the bristle-blasted surface.

Where repeatability across large areas or shifts matters most, automation goes a step further. The Prepper® Q10 is programmed for the desired roughness and is designed to deliver a regular peak height and peak density, and it can be integrated into factory-applied coating processes.

Frequently asked questions

What is peak density in surface profile?

Peak density is the number of profile peaks in a given area of the prepared surface, usually reported as peaks per mm². The related stylus parameter, relative peak count (Rpc), counts peak and valley pairs per unit length, usually in peaks per cm. Both describe how closely spaced the anchor points are, which peak height alone does not.

Does a higher surface profile always give better adhesion?

No. Peak height must suit the coating: too low gives too little anchorage, too high can leave peaks poorly covered. At a given height, more peaks per unit area increase the surface area the coating bonds to. Roper, Weaver and Brandon (2005) found that a high peak count helps resist undercutting corrosion and improves adhesion.

How do you measure peak density on site?

Peak count can be measured with a portable stylus instrument, formerly under ASTM D7127, which was withdrawn in 2021 and folded into ASTM D4417. Peak density can also be read from optical grade replica tape with a suitable reader, which reports peak height and peak density from the same replica.

Should a surface preparation specification include peak density?

Where the coating manufacturer or the asset owner gives a peak density or peak count requirement, yes, together with the measurement method, number of readings and acceptance criteria, for example per SSPC-PA 17. Where none is given, specify the profile height range from the data sheet and a preparation process that is repeatable.