TSS Membership Community

Dear All,

I have two questions regarding the blasting and preheating of samples prior to thermal spraying. I would be grateful if you could share your experience and wisdom with me.

• Is there any standard for the time limit between the blasting and spraying operations to prevent the formation of an oxide layer on the blasted surfaces?
• Is it a MUST to preheat the samples for at least one pass to eliminate the negative impact of humidity on the adhesion between the coating and the substrate? Is there any standard for that?

Thanks in advance for your attention.

------------------------------
Milad Rad, Ph.D.
Assistant Professor
University of Southern Indiana
------------------------------

Hi Milad,

These are very important questions.

Regarding grit-blasting; I have never seen a general "open-literature" standard limiting the maximum delay between the grit-blasting and spraying deposition of a component. But most likely institutions/shops have their own internal guidelines about the maximum time limit a part can stay blasted before spraying. As a "rule of thumb", the spraying step begins no more than 1-h after the part was grit-blasted. 

Regarding part/substrate pre-heating; it is not a must to pre-heat the sample before spraying. It depends on the size of the part, among other factors. What I mean is, sometimes the part is so large that when you finish the 1st pre-heat pass, the other extreme of it is already at RT (e.g., large cylinder rolls). That said, for some types of components, a pre-heating step before the spraying of the part is a must. For example, a porous cast part that was in contact with a fluid; like a pump casing. In these porous castings of pumps, a fluid/liquid can penetrate into the micro-porosities/crevices of the cast. Therefore, if you just girt-blast the part without performing any preheating, that infiltrated liquid will tend to come out during spraying; thereby causing poor coating adhesion and even coating spallation. In this case, you need to grit-blast the part (to remove deposits) and then use a large blow torch to to heat it up. You can see with your own eyes that the part begins "to sweat". It is the infiltrated liquid coming out to the surface. After this process is done, the part is grit-blasted again and the spraying is performed, but without part pre-heating. Another area in which part pre-heating seems to be highly important is to spray polymers. That pre-heating is necessary for enhancing the adhesion.

Again... these are interesting questions. Hopefully someone with a stronger industrial experience will contribute to this discussion.

I hope it helps!

------------------------------
Rogerio Lima, PhD
Senior Research Officer
National Research Council of Canada
ASM Thermal Spray Society (TSS) Vice President
------------------------------

Original Message:
Sent: 08-21-2022 10:27
From: Milad Rad
Subject: Standards for Blasting and Preheating

Dear All,

I have two questions regarding the blasting and preheating of samples prior to thermal spraying. I would be grateful if you could share your experience and wisdom with me.

• Is there any standard for the time limit between the blasting and spraying operations to prevent the formation of an oxide layer on the blasted surfaces?
• Is it a MUST to preheat the samples for at least one pass to eliminate the negative impact of humidity on the adhesion between the coating and the substrate? Is there any standard for that?

Thanks in advance for your attention.

------------------------------
Milad Rad, Ph.D.
Assistant Professor
University of Southern Indiana
------------------------------

Hi Rogerio,

Thank you very much for sharing your knowledge with me. I found your explanation very helpful. I do appreciate your informative response and valuable feedback. 

------------------------------
Milad Rad, Ph.D.
Assistant Professor
University of Southern Indiana
------------------------------

Original Message:
Sent: 08-22-2022 17:16
From: Rogerio Lima
Subject: Standards for Blasting and Preheating

Hi Milad,

These are very important questions.

Regarding grit-blasting; I have never seen a general "open-literature" standard limiting the maximum delay between the grit-blasting and spraying deposition of a component. But most likely institutions/shops have their own internal guidelines about the maximum time limit a part can stay blasted before spraying. As a "rule of thumb", the spraying step begins no more than 1-h after the part was grit-blasted. 

Regarding part/substrate pre-heating; it is not a must to pre-heat the sample before spraying. It depends on the size of the part, among other factors. What I mean is, sometimes the part is so large that when you finish the 1st pre-heat pass, the other extreme of it is already at RT (e.g., large cylinder rolls). That said, for some types of components, a pre-heating step before the spraying of the part is a must. For example, a porous cast part that was in contact with a fluid; like a pump casing. In these porous castings of pumps, a fluid/liquid can penetrate into the micro-porosities/crevices of the cast. Therefore, if you just girt-blast the part without performing any preheating, that infiltrated liquid will tend to come out during spraying; thereby causing poor coating adhesion and even coating spallation. In this case, you need to grit-blast the part (to remove deposits) and then use a large blow torch to to heat it up. You can see with your own eyes that the part begins "to sweat". It is the infiltrated liquid coming out to the surface. After this process is done, the part is grit-blasted again and the spraying is performed, but without part pre-heating. Another area in which part pre-heating seems to be highly important is to spray polymers. That pre-heating is necessary for enhancing the adhesion.

Again... these are interesting questions. Hopefully someone with a stronger industrial experience will contribute to this discussion.

I hope it helps!

------------------------------
Rogerio Lima, PhD
Senior Research Officer
National Research Council of Canada
ASM Thermal Spray Society (TSS) Vice President

Original Message:
Sent: 08-21-2022 10:27
From: Milad Rad
Subject: Standards for Blasting and Preheating

Dear All,

I have two questions regarding the blasting and preheating of samples prior to thermal spraying. I would be grateful if you could share your experience and wisdom with me.

• Is there any standard for the time limit between the blasting and spraying operations to prevent the formation of an oxide layer on the blasted surfaces?
• Is it a MUST to preheat the samples for at least one pass to eliminate the negative impact of humidity on the adhesion between the coating and the substrate? Is there any standard for that?

Thanks in advance for your attention.

------------------------------
Milad Rad, Ph.D.
Assistant Professor
University of Southern Indiana
------------------------------